diff --git a/nomenclature/codelist.py b/nomenclature/codelist.py
index 1233a2a4..712481e9 100644
--- a/nomenclature/codelist.py
+++ b/nomenclature/codelist.py
@@ -390,7 +390,7 @@ def increase_indent(self, flow: bool = False, indentless: bool = False):
 
         if path is None:
             return stream
-        with open(path, "w") as file:
+        with open(path, "w", encoding="utf-8") as file:
             file.write(stream)
 
     def to_pandas(self, sort_by_code: bool = False) -> pd.DataFrame:
@@ -681,7 +681,7 @@ def list_missing_variables(
                 },
             ).to_yaml()
 
-            with open(file, "a") as f:
+            with open(file, "a", encoding="utf-8") as f:
                 f.write(missing_variables_formatted)
 
 
diff --git a/nomenclature/config.py b/nomenclature/config.py
index 0bae16b8..b5a59b7a 100644
--- a/nomenclature/config.py
+++ b/nomenclature/config.py
@@ -101,7 +101,7 @@ def fetch_repo(self, to_path):
     def check_external_repo_double_stacking(self):
         nomenclature_config = self.local_path / "nomenclature.yaml"
         if nomenclature_config.is_file():
-            with open(nomenclature_config, "r") as f:
+            with open(nomenclature_config, "r", encoding="utf-8") as f:
                 config = yaml.safe_load(f)
             if config.get("repositories"):
                 raise ValueError(
diff --git a/nomenclature/processor/data_validator.py b/nomenclature/processor/data_validator.py
index 17b9624f..c0731aac 100644
--- a/nomenclature/processor/data_validator.py
+++ b/nomenclature/processor/data_validator.py
@@ -85,7 +85,7 @@ def check_criteria(cls, v):
 
     @classmethod
     def from_file(cls, file: Union[Path, str]) -> "DataValidator":
-        with open(file, "r") as f:
+        with open(file, "r", encoding="utf-8") as f:
             content = yaml.safe_load(f)
         return cls(file=file, criteria_items=content)
 
diff --git a/nomenclature/processor/region.py b/nomenclature/processor/region.py
index a2da27ba..ce7ba43d 100644
--- a/nomenclature/processor/region.py
+++ b/nomenclature/processor/region.py
@@ -268,7 +268,7 @@ def from_file(cls, file: Union[Path, str]):
     @classmethod
     def from_yaml(cls, file: Path) -> "RegionAggregationMapping":
         try:
-            with open(file, "r") as f:
+            with open(file, "r", encoding="utf-8") as f:
                 mapping_input = yaml.safe_load(f)
 
             # Add the file name to mapping_input
@@ -425,7 +425,7 @@ def to_yaml(self, file) -> None:
             ]
         if self.exclude_regions:
             dict_representation["exclude_regions"] = self.exclude_regions
-        with open(file, "w") as f:
+        with open(file, "w", encoding="utf-8") as f:
             yaml.dump(dict_representation, f, sort_keys=False)
 
 
diff --git a/nomenclature/processor/required_data.py b/nomenclature/processor/required_data.py
index 65643bd9..76d124c2 100644
--- a/nomenclature/processor/required_data.py
+++ b/nomenclature/processor/required_data.py
@@ -159,7 +159,7 @@ def convert_to_list(cls, v):
 
     @classmethod
     def from_file(cls, file: Union[Path, str]) -> "RequiredDataValidator":
-        with open(file, "r") as f:
+        with open(file, "r", encoding="utf-8") as f:
             content = yaml.safe_load(f)
         return cls(file=file, **content)
 
diff --git a/tests/test_definition.py b/tests/test_definition.py
index 5243c64d..8700738f 100644
--- a/tests/test_definition.py
+++ b/tests/test_definition.py
@@ -132,9 +132,10 @@ def test_create_yaml_from_xlsx(input_file, attrs, exp_file, tmpdir):
         attrs=attrs,
     )
 
-    with open(file, "r") as f:
+    with open(file, "r", encoding="utf-8") as f:
         obs = f.read()
-    with open(TEST_DATA_DIR / "io" / "excel_io" / exp_file, "r") as f:
+    with open(TEST_DATA_DIR / "io" / "excel_io" / exp_file, "r",
+              encoding="utf-8") as f:
         exp = f.read()
 
     assert obs == exp
diff --git a/tests/test_model_registration_parser.py b/tests/test_model_registration_parser.py
index b446f740..c9142525 100644
--- a/tests/test_model_registration_parser.py
+++ b/tests/test_model_registration_parser.py
@@ -15,7 +15,8 @@ def test_parse_model_registration(tmp_path):
     )
 
     # Test model mapping
-    with open(tmp_path / "Model 1.1_mapping.yaml", "r") as file:
+    with open(tmp_path / "Model 1.1_mapping.yaml", "r", encoding="utf-8") \
+            as file:
         obs_model_mapping = yaml.safe_load(file)
     with open(
         TEST_DATA_DIR
@@ -23,12 +24,14 @@ def test_parse_model_registration(tmp_path):
         / "region_aggregation"
         / "excel_mapping_reference.yaml",
         "r",
+        encoding="utf-8",
     ) as file:
         exp_model_mapping = yaml.safe_load(file)
     assert obs_model_mapping == exp_model_mapping
 
     # Test model regions
-    with open(tmp_path / "Model 1.1_regions.yaml", "r") as file:
+    with open(tmp_path / "Model 1.1_regions.yaml", "r", encoding="utf-8") \
+            as file:
         obs_model_regions = yaml.safe_load(file)
     exp_model_regions = [
         {"Model 1.1": ["Model 1.1|Region 1", "Region 2", "Model 1.1|Region 3"]}