diff --git a/premailer/merge_style.py b/premailer/merge_style.py
index 62d4a9d..5fc29d9 100644
--- a/premailer/merge_style.py
+++ b/premailer/merge_style.py
@@ -61,10 +61,13 @@ def merge_styles(inline_style, new_styles, classes, remove_unset_properties=Fals
 
     # keep always the old inline style
     if inline_style:
+        normal_styles_dict = styles[""]
         # inline should be a declaration list as I understand
         # ie property-name:property-value;...
         for k, v in csstext_to_pairs(inline_style):
-            styles[""][k] = v
+            if k in normal_styles_dict:
+                del normal_styles_dict[k]
+            normal_styles_dict[k] = v
 
     normal_styles = []
     pseudo_styles = []
diff --git a/premailer/tests/test_merge_style.py b/premailer/tests/test_merge_style.py
index 4b13e7e..14fd4ff 100644
--- a/premailer/tests/test_merge_style.py
+++ b/premailer/tests/test_merge_style.py
@@ -15,3 +15,15 @@ def test_inline_invalid_syntax(self):
         # Invalid syntax does not raise
         inline = "{color:pink} :hover{color:purple} :active{color:red}"
         merge_styles(inline, [], [])
+
+    def test_constituent_styles(self):
+        # "constituent": `margin-bottom` is a constituent style of `margin`
+        new_styles = [[("margin", "5px"), ("margin-bottom", "10px")]]
+        classes = [""]
+        inline_style = "margin: 0"
+        csstext = merge_styles(inline_style, new_styles, classes)
+        self.assertEqual(
+            # ideally premailer could eliminate margin-bottom altogether
+            [("margin-bottom", "10px"), ("margin", "0")],
+            csstext_to_pairs(csstext),
+        )