diff --git a/src/epoint/__init__.py b/src/epoint/__init__.py
index e553140..c27c70f 100644
--- a/src/epoint/__init__.py
+++ b/src/epoint/__init__.py
@@ -3,19 +3,43 @@
 from .aio import AsyncEpointClient
 from .callbacks import verify_callback
 from .client import EpointClient
+from .enums import (
+    AZN_ONLY,
+    SETTLED_STATUSES,
+    SUPPORTED_CURRENCIES,
+    USABLE_CARD_STATUSES,
+    B2BStatus,
+    CardStatus,
+    Currency,
+    InvoiceStatus,
+    Language,
+    OperationCode,
+    Status,
+)
 from .errors import EpointError, GatewayError, SignatureError, TransportError
 from .models import Callback, Response
 
 __version__ = "0.1.1"
 
 __all__ = [
+    "AZN_ONLY",
+    "SETTLED_STATUSES",
+    "SUPPORTED_CURRENCIES",
+    "USABLE_CARD_STATUSES",
     "AsyncEpointClient",
+    "B2BStatus",
     "Callback",
+    "CardStatus",
+    "Currency",
     "EpointClient",
     "EpointError",
     "GatewayError",
+    "InvoiceStatus",
+    "Language",
+    "OperationCode",
     "Response",
     "SignatureError",
+    "Status",
     "TransportError",
     "__version__",
     "verify_callback",
diff --git a/src/epoint/_transport.py b/src/epoint/_transport.py
index b05a15a..344be03 100644
--- a/src/epoint/_transport.py
+++ b/src/epoint/_transport.py
@@ -3,6 +3,7 @@
 from typing import Any
 
 from . import _signing
+from .enums import Status
 from .errors import GatewayError, TransportError
 from .models import Response
 
@@ -21,7 +22,7 @@ def parse(status_code: int, body: Any) -> Response:
     if not isinstance(body, dict):
         raise TransportError(f"unexpected response body: {body!r}", status_code=status_code)
 
-    if body.get("status") == "error":
+    if body.get("status") == Status.ERROR:
         raise GatewayError(
             str(body.get("message") or "request refused"),
             code=str(body["code"]) if body.get("code") is not None else None,
diff --git a/src/epoint/aio.py b/src/epoint/aio.py
index 41b7341..7dab58f 100644
--- a/src/epoint/aio.py
+++ b/src/epoint/aio.py
@@ -11,6 +11,7 @@
 from ._endpoints import Amount, Defaults
 from .callbacks import verify_callback
 from .client import PRODUCTION_URL
+from .enums import Currency, Language
 from .errors import TransportError
 from .models import Callback, Response
 
@@ -22,8 +23,8 @@ def __init__(
         private_key: str,
         *,
         base_url: str = PRODUCTION_URL,
-        language: str = "az",
-        currency: str = "AZN",
+        language: str = Language.AZ,
+        currency: str = Currency.AZN,
         success_redirect_url: str | None = None,
         error_redirect_url: str | None = None,
         timeout: float = 30.0,
@@ -42,7 +43,7 @@ def __init__(
     def from_env(cls, **overrides: Any) -> AsyncEpointClient:
         params: dict[str, Any] = {
             "base_url": os.environ.get("EPOINT_BASE_URL", PRODUCTION_URL),
-            "language": os.environ.get("EPOINT_LANGUAGE", "az"),
+            "language": os.environ.get("EPOINT_LANGUAGE", Language.AZ),
             "success_redirect_url": os.environ.get("EPOINT_SUCCESS_REDIRECT_URL"),
             "error_redirect_url": os.environ.get("EPOINT_FAILED_REDIRECT_URL"),
         }
diff --git a/src/epoint/client.py b/src/epoint/client.py
index 53fcc42..cc6246e 100644
--- a/src/epoint/client.py
+++ b/src/epoint/client.py
@@ -10,6 +10,7 @@
 from . import _transport
 from ._endpoints import Amount, Defaults
 from .callbacks import verify_callback
+from .enums import Currency, Language
 from .errors import TransportError
 from .models import Callback, Response
 
@@ -23,8 +24,8 @@ def __init__(
         private_key: str,
         *,
         base_url: str = PRODUCTION_URL,
-        language: str = "az",
-        currency: str = "AZN",
+        language: str = Language.AZ,
+        currency: str = Currency.AZN,
         success_redirect_url: str | None = None,
         error_redirect_url: str | None = None,
         timeout: float = 30.0,
@@ -43,7 +44,7 @@ def __init__(
     def from_env(cls, **overrides: Any) -> EpointClient:
         params: dict[str, Any] = {
             "base_url": os.environ.get("EPOINT_BASE_URL", PRODUCTION_URL),
-            "language": os.environ.get("EPOINT_LANGUAGE", "az"),
+            "language": os.environ.get("EPOINT_LANGUAGE", Language.AZ),
             "success_redirect_url": os.environ.get("EPOINT_SUCCESS_REDIRECT_URL"),
             "error_redirect_url": os.environ.get("EPOINT_FAILED_REDIRECT_URL"),
         }
diff --git a/src/epoint/enums.py b/src/epoint/enums.py
new file mode 100644
index 0000000..86ceb10
--- /dev/null
+++ b/src/epoint/enums.py
@@ -0,0 +1,82 @@
+from __future__ import annotations
+
+from enum import Enum
+
+
+class StrEnum(str, Enum):
+    """A str subclass, so members go over the wire unchanged.
+
+    Python 3.11 has this built in. Defining it here keeps 3.10 working.
+    """
+
+    __hash__ = str.__hash__
+
+    def __str__(self) -> str:
+        return str(self.value)
+
+
+class Status(StrEnum):
+    """The status on a response or a callback."""
+
+    NEW = "new"
+    SUCCESS = "success"
+    FAILED = "failed"
+    ERROR = "error"
+    RETURNED = "returned"
+    SERVER_ERROR = "server_error"
+
+
+SETTLED_STATUSES = frozenset({Status.SUCCESS.value})
+
+
+class CardStatus(StrEnum):
+    NEW = "new"
+    ACTIVE = "active"
+    PENDING = "pending"
+    REJECTED = "rejected"
+    EXPIRED = "expired"
+    SESSION_EXPIRED = "session_expired"
+
+
+USABLE_CARD_STATUSES = frozenset({CardStatus.ACTIVE.value})
+
+
+class InvoiceStatus(StrEnum):
+    WAITING = "waiting_for_payment"
+    PAID = "paid"
+    CANCELED = "canceled"
+
+
+class B2BStatus(StrEnum):
+    PENDING = "PENDING"
+    PROCESSING = "PROCESSING"
+    SUCCESS = "SUCCESS"
+    FAILED = "FAILED"
+
+
+class OperationCode(StrEnum):
+    """What the transaction did, as it comes back on a callback."""
+
+    CARD_REGISTRATION = "001"
+    PAYMENT = "100"
+    REGISTRATION_WITH_PAYMENT = "200"
+
+
+class Language(StrEnum):
+    AZ = "az"
+    EN = "en"
+    RU = "ru"
+
+
+class Currency(StrEnum):
+    AZN = "AZN"
+    USD = "USD"
+    EUR = "EUR"
+    RUB = "RUB"
+
+
+#: Checkout takes any of these.
+SUPPORTED_CURRENCIES = frozenset(c.value for c in Currency)
+
+#: Split, pre-auth, refund, reverse, payout and wallet take AZN and nothing else.
+AZN_ONLY = frozenset({Currency.AZN.value})
diff --git a/src/epoint/models.py b/src/epoint/models.py
index 86e1538..14ca547 100644
--- a/src/epoint/models.py
+++ b/src/epoint/models.py
@@ -3,6 +3,8 @@
 from dataclasses import dataclass
 from typing import Any
 
+from .enums import SETTLED_STATUSES
+
 
 class Response:
     def __init__(self, raw: dict[str, Any]) -> None:
@@ -15,7 +17,7 @@ def status(self) -> str | None:
 
     @property
     def ok(self) -> bool:
-        return self.raw.get("status") == "success"
+        return self.raw.get("status") in SETTLED_STATUSES
 
     @property
     def transaction(self) -> str | None:
@@ -63,7 +65,7 @@ class Callback:
 
     @property
     def ok(self) -> bool:
-        return self.status == "success"
+        return self.status in SETTLED_STATUSES
 
     @classmethod
     def from_dict(cls, data: dict[str, Any]) -> Callback:
diff --git a/tests/test_client_unit.py b/tests/test_client_unit.py
index 5cd91b7..478f416 100644
--- a/tests/test_client_unit.py
+++ b/tests/test_client_unit.py
@@ -148,3 +148,20 @@ def handler(request: httpx.Request) -> httpx.Response:
     assert "application/json" in seen["content_type"]
     assert "data" in seen["body"]
     assert "signature" in seen["body"]
+
+
+def test_enums_go_over_the_wire_as_plain_strings():
+    from epoint import Currency, Language, OperationCode, Status
+
+    assert json.dumps({"language": Language.AZ, "currency": Currency.AZN}) == (
+        '{"language": "az", "currency": "AZN"}'
+    )
+    assert OperationCode.PAYMENT == "100"
+    assert "success" in {Status.SUCCESS, Status.NEW}
+
+
+def test_a_raw_status_string_still_reads_as_ok():
+    from epoint.models import Response
+
+    assert Response({"status": "success"}).ok
+    assert not Response({"status": "failed"}).ok
diff --git a/tests/test_sandbox.py b/tests/test_sandbox.py
index ca64867..8d06a30 100644
--- a/tests/test_sandbox.py
+++ b/tests/test_sandbox.py
@@ -3,7 +3,7 @@
 import httpx
 import pytest
 
-from epoint import EpointClient, GatewayError
+from epoint import CardStatus, EpointClient, GatewayError, Status
 
 pytestmark = pytest.mark.sandbox
 
@@ -144,3 +144,28 @@ def test_missing_required_field_is_refused(client):
     with pytest.raises(GatewayError) as info:
         client._post("/api/1/request", {"amount": "10.00", "currency": "AZN"})
     assert "order_id" in str(info.value)
+
+
+def test_the_statuses_the_sandbox_returns_are_all_in_the_enum(client, pay):
+    """Guards against the enum drifting away from what the API actually sends."""
+    paid = client.create_payment(amount=5, order_id=order())
+    pay(client, paid.redirect_url)
+
+    declined = client.create_payment(amount=5, order_id=order())
+    pay(client, declined.redirect_url, card="4000000000000116")
+
+    seen = {
+        client.get_status(paid.transaction).status,
+        client.get_status(declined.transaction).status,
+    }
+    assert seen == {Status.SUCCESS, Status.FAILED}
+    assert all(s in set(Status) for s in seen)
+
+
+def test_a_card_status_from_the_sandbox_is_in_the_enum(client, pay):
+    registered = client.register_card(description="enum check")
+    pay(client, registered.redirect_url)
+
+    card = client.get_card_status(registered["card_id"])
+    assert card.get("status") == CardStatus.ACTIVE
+    assert card.get("status") in set(CardStatus)