diff --git a/altamisa/isatab/parse_assay_study.py b/altamisa/isatab/parse_assay_study.py
index 9947df6..a49ac97 100644
--- a/altamisa/isatab/parse_assay_study.py
+++ b/altamisa/isatab/parse_assay_study.py
@@ -768,6 +768,9 @@ def _read_next_line(self):
                 self.unique_rows.add("\t".join(self._line))
         except StopIteration:
             self._line = None
+        except UnicodeDecodeError as e:  # pragma: no cover
+            msg = f"Invalid encoding of study file '{self._filename}' (use Unicode/UTF-8)."
+            raise ParseIsatabException(msg) from e
         return prev_line
 
     def read(self):
@@ -888,6 +891,9 @@ def _read_next_line(self):
                 self.unique_rows.add("\t".join(self._line))
         except StopIteration:
             self._line = None
+        except UnicodeDecodeError as e:  # pragma: no cover
+            msg = f"Invalid encoding of assay file '{self._filename}' (use Unicode/UTF-8)."
+            raise ParseIsatabException(msg) from e
         return prev_line
 
     def read(self):
diff --git a/altamisa/isatab/parse_investigation.py b/altamisa/isatab/parse_investigation.py
index 5ac8aff..a365cbf 100644
--- a/altamisa/isatab/parse_investigation.py
+++ b/altamisa/isatab/parse_investigation.py
@@ -132,6 +132,9 @@ def _read_next_line(self):
                 self._line = list_strip(next(self._reader))
         except StopIteration:
             self._line = None
+        except UnicodeDecodeError as e:  # pragma: no cover
+            msg = f"Invalid encoding of investigation file '{self._filename}' (use Unicode/UTF-8)."
+            raise ParseIsatabException(msg) from e
         return prev_line
 
     def _next_line_startswith_comment(self):