diff --git a/src/ert/config/ensemble_config.py b/src/ert/config/ensemble_config.py
index edcae8e6f13..2647d4bec59 100644
--- a/src/ert/config/ensemble_config.py
+++ b/src/ert/config/ensemble_config.py
@@ -3,7 +3,7 @@
 import logging
 import os
 from collections import Counter
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from datetime import datetime
 from typing import (
     Any,
@@ -72,23 +72,20 @@ def all_dates(self) -> List[datetime]:
         return [self.start_date] + list(self.dates)
 
 
+@dataclass
 class EnsembleConfig:
-    def __init__(
-        self,
-        grid_file: Optional[str] = None,
-        response_configs: Optional[Dict[str, ResponseConfig]] = None,
-        parameter_configs: Optional[Dict[str, ParameterConfig]] = None,
-        refcase: Optional[Refcase] = None,
-    ) -> None:
-        parameter_configs = parameter_configs if parameter_configs is not None else {}
-        response_configs = response_configs if response_configs is not None else {}
+    grid_file: Optional[str] = None
+    response_configs: Optional[Dict[str, ResponseConfig]] = field(default_factory=dict)
+    parameter_configs: Optional[Dict[str, ParameterConfig]] = field(
+        default_factory=dict
+    )
+    refcase: Optional[Refcase] = None
+
+    def __post_init__(self):
         self._check_for_duplicate_names(
-            list(parameter_configs.values()), list(response_configs.values())
+            list(self.parameter_configs.values()), list(self.response_configs.values())
         )
-        self.parameter_configs = parameter_configs
-        self.response_configs = response_configs
-        self._grid_file = _get_abs_path(grid_file)
-        self.refcase = refcase
+        self.grid_file = _get_abs_path(self.grid_file)
 
     @staticmethod
     def _check_for_duplicate_names(
@@ -225,10 +222,6 @@ def get_keylist_gen_kw(self) -> List[str]:
             if isinstance(val, GenKwConfig)
         ]
 
-    @property
-    def grid_file(self) -> Optional[str]:
-        return self._grid_file
-
     @property
     def parameters(self) -> List[str]:
         return list(self.parameter_configs)
@@ -244,18 +237,6 @@ def keys(self) -> List[str]:
     def __contains__(self, key: str) -> bool:
         return key in self.keys
 
-    def __eq__(self, other: object) -> bool:
-        if not isinstance(other, EnsembleConfig):
-            return False
-
-        return (
-            self.keys == other.keys
-            and self._grid_file == other._grid_file
-            and self.parameter_configs == other.parameter_configs
-            and self.response_configs == other.response_configs
-            and self.refcase == other.refcase
-        )
-
     @property
     def parameter_configuration(self) -> List[ParameterConfig]:
         return list(self.parameter_configs.values())
diff --git a/src/ert/config/ert_config.py b/src/ert/config/ert_config.py
index 5998b404789..fdae03ab607 100644
--- a/src/ert/config/ert_config.py
+++ b/src/ert/config/ert_config.py
@@ -105,8 +105,8 @@ class ErtConfig:
     user_config_file: str = "no_config"
     config_path: str = field(init=False)
     observation_config: Optional[
-        Dict[str, Union[HistoryValues, SummaryValues, GenObsValues]]
-    ] = None
+        List[Tuple[str, Union[HistoryValues, SummaryValues, GenObsValues]]]
+    ] = field(default_factory=list)
 
     def __eq__(self, other: object) -> bool:
         if not isinstance(other, ErtConfig):
diff --git a/src/ert/config/observations.py b/src/ert/config/observations.py
index 44e42184b81..fda323bb561 100644
--- a/src/ert/config/observations.py
+++ b/src/ert/config/observations.py
@@ -1,4 +1,5 @@
 import os
+from dataclasses import dataclass
 from datetime import datetime, timedelta
 from typing import TYPE_CHECKING, Dict, Iterator, List, Optional, Tuple, Union
 
@@ -35,10 +36,12 @@ def history_key(key: str) -> str:
     return ":".join([keyword + "H"] + rest)
 
 
+@dataclass
 class EnkfObs:
-    def __init__(self, obs_vectors: Dict[str, ObsVector], obs_time: List[datetime]):
-        self.obs_vectors = obs_vectors
-        self.obs_time = obs_time
+    obs_vectors: Dict[str, ObsVector]
+    obs_time: List[datetime]
+
+    def __post_init__(self):
         self.datasets: Dict[str, xr.Dataset] = {
             name: obs.to_dataset([]) for name, obs in sorted(self.obs_vectors.items())
         }
diff --git a/tests/unit_tests/config/test_ert_config.py b/tests/unit_tests/config/test_ert_config.py
index 9c4eb565dc6..40083dd5fcc 100644
--- a/tests/unit_tests/config/test_ert_config.py
+++ b/tests/unit_tests/config/test_ert_config.py
@@ -1558,7 +1558,7 @@ def test_that_multiple_errors_are_shown_when_validating_observation_config():
                 continue
             print(line, end="")
 
-    with pytest.raises(ObservationConfigError) as err:
+    with pytest.raises(ConfigValidationError) as err:
         _ = ErtConfig.from_file("snake_oil.ert")
 
     expected_errors = [