diff --git a/bibtexparser/middlewares/interpolate.py b/bibtexparser/middlewares/interpolate.py
index 2e5d832..0842f9d 100644
--- a/bibtexparser/middlewares/interpolate.py
+++ b/bibtexparser/middlewares/interpolate.py
@@ -21,6 +21,72 @@ def _value_is_nonstring_or_enclosed(value: Any) -> bool:
     return False
 
 
+def _split_concatenation(value: str) -> "list[str] | None":
+    """Split a value on top-level ``#`` concatenation operators.
+
+    Returns the list of stripped tokens if the value contains at least one
+    ``#`` outside of any ``"..."`` or ``{...}`` group, otherwise ``None``
+    (i.e., the value is not a concatenation expression).
+    """
+    tokens = []
+    current = []
+    depth = 0
+    in_quotes = False
+    found_separator = False
+    for char in value:
+        if char == '"' and depth == 0:
+            in_quotes = not in_quotes
+            current.append(char)
+        elif char == "{" and not in_quotes:
+            depth += 1
+            current.append(char)
+        elif char == "}" and not in_quotes:
+            depth = max(0, depth - 1)
+            current.append(char)
+        elif char == "#" and depth == 0 and not in_quotes:
+            found_separator = True
+            tokens.append("".join(current).strip())
+            current = []
+        else:
+            current.append(char)
+
+    if not found_separator:
+        return None
+
+    tokens.append("".join(current).strip())
+    return tokens
+
+
+def _resolve_concatenation(tokens: "list[str]", string_values: "dict[str, str]") -> "str | None":
+    """Resolve concatenation tokens to their joined string content.
+
+    Each token is a number (kept verbatim), a quoted or braced string (its
+    inner content is used), or a string reference (resolved via
+    ``string_values``). Returns ``None`` if any reference is unknown, so the
+    caller can leave the original expression untouched.
+    """
+    resolved = []
+    for token in tokens:
+        if not token:
+            return None
+        if token.startswith('"') and token.endswith('"'):
+            resolved.append(token[1:-1])
+        elif token.startswith("{") and token.endswith("}"):
+            resolved.append(token[1:-1])
+        elif token.isdigit():
+            resolved.append(token)
+        else:
+            try:
+                referenced = string_values[token.lower()]
+            except KeyError:
+                return None
+            if referenced.startswith(('"', "{")) and referenced.endswith(('"', "}")):
+                referenced = referenced[1:-1]
+            resolved.append(referenced)
+
+    return "".join(resolved)
+
+
 class ResolveStringReferencesMiddleware(LibraryMiddleware):
     """Replace strings references with their values."""
 
@@ -59,6 +125,18 @@ def transform(self, library: Library) -> Library:
 
             field: Field
             for field in entry.fields:
+                if isinstance(field.value, str):
+                    tokens = _split_concatenation(field.value)
+                    if tokens is not None:
+                        joined = _resolve_concatenation(tokens, string_values)
+                        if joined is not None:
+                            # Keep the result enclosed (in braces) so the
+                            # downstream enclosing middlewares treat it as a
+                            # plain string rather than an unenclosed reference.
+                            field.value = "{" + joined + "}"
+                            resolved_fields.append(field.key)
+                        continue
+
                 if _value_is_nonstring_or_enclosed(field.value):
                     continue
                 try:
diff --git a/tests/middleware_tests/test_interpolate.py b/tests/middleware_tests/test_interpolate.py
index 264373c..a223598 100644
--- a/tests/middleware_tests/test_interpolate.py
+++ b/tests/middleware_tests/test_interpolate.py
@@ -1,5 +1,6 @@
 import pytest
 
+import bibtexparser
 from bibtexparser.middlewares.enclosing import RemoveEnclosingMiddleware
 from bibtexparser.middlewares.interpolate import ResolveStringReferencesMiddleware
 from bibtexparser.splitter import Splitter
@@ -66,3 +67,65 @@ def test_warning_is_raised_if_enclosings_are_removed():
 
     assert len(record) == 1
     assert "RemoveEnclosing" in record[0].message.args[0]
+
+
+def test_string_interpolation_resolves_concatenation():
+    bibtex = """
+    @string{jan = "Jan."}
+
+    @inbook{test_inbook,
+      month   = 10 # "~" # jan,
+      title   = "hello" # " " # "world",
+      mixed   = jan # {  } # "X",
+      numbers = 1 # 2 # 3
+    }
+    """
+    library = Splitter(bibtex).split()
+
+    m = ResolveStringReferencesMiddleware()
+    library = m.transform(library)
+
+    fields = library.entries_dict["test_inbook"].fields_dict
+    # Concatenations are resolved and kept enclosed in braces.
+    assert fields["month"].value == "{10~Jan.}"
+    assert fields["title"].value == "{hello world}"
+    assert fields["mixed"].value == "{Jan.  X}"
+    assert fields["numbers"].value == "{123}"
+
+
+def test_string_interpolation_leaves_unresolvable_concatenation_untouched():
+    bibtex = """
+    @inbook{test_inbook,
+      note = unknown_macro # " suffix"
+    }
+    """
+    library = Splitter(bibtex).split()
+
+    m = ResolveStringReferencesMiddleware()
+    library = m.transform(library)
+
+    # An unknown reference means the expression is left exactly as-is.
+    assert (
+        library.entries_dict["test_inbook"].fields_dict["note"].value == 'unknown_macro # " suffix"'
+    )
+
+
+def test_parse_string_resolves_concatenation_end_to_end():
+    # Exact reproduction of issue #396.
+    bibtex_str = """
+    @STRING{ jan = "Jan." }
+
+    @INBOOK{inbook-full,
+       month = 10 # "~" # jan,
+    }
+    """
+    library = bibtexparser.parse_string(bibtex_str)
+    month = library.entries[0].fields_dict["month"].value
+    assert month == "10~Jan."
+
+    # The resolved value writes as a valid, brace-enclosed string and
+    # round-trips back to the same value.
+    written = bibtexparser.write_string(library)
+    assert "month = {10~Jan.}" in written
+    reparsed = bibtexparser.parse_string(written)
+    assert reparsed.entries[0].fields_dict["month"].value == "10~Jan."