diff --git a/CHANGELOG.md b/CHANGELOG.md
index cad8eb9..9fcd3a6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file.
 
+## v0.4.0
+
+### Changed
+
+- Made `YearMonth` instances immutable.
+
 ## v0.3.1
 
 ### Changed
diff --git a/mp_yearmonth/yearmonth.py b/mp_yearmonth/yearmonth.py
index cd2c3c0..b0852f6 100644
--- a/mp_yearmonth/yearmonth.py
+++ b/mp_yearmonth/yearmonth.py
@@ -41,7 +41,7 @@ class YearMonth:
         month (int) : The month.
     """
 
-    __slots__ = ("year", "month")
+    __slots__ = ("_year", "_month")
 
     def __init__(self, year: int, month: int):
         """Create a new YearMonth object.
@@ -63,8 +63,8 @@ def __init__(self, year: int, month: int):
                 f"year must be between {datetime.MINYEAR} and {datetime.MAXYEAR}"
             )
 
-        self.year = year
-        self.month = month
+        self._year = year
+        self._month = month
 
     def __str__(self):
         return self.iso8601
@@ -117,6 +117,16 @@ def __contains__(self, other) -> bool:
 
     # Public interface
 
+    @property
+    def year(self) -> int:
+        """The year."""
+        return self._year
+
+    @property
+    def month(self) -> int:
+        """The month."""
+        return self._month
+
     @property
     def iso8601(self) -> str:
         """The ISO 8601 representation of the year and month."""
diff --git a/pyproject.toml b/pyproject.toml
index 1500082..c8a1cca 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "mp-yearmonth"
-version = "0.3.1"
+version = "0.4.0"
 description = "A year-month datatype for Python."
 keywords = ["year", "month", "date", "calendar"]
 authors = ["Ramon Torres <ramon@macropoetry.com>"]