diff --git a/src/buildMathML.js b/src/buildMathML.js
index 20d42560..89aa6e58 100644
--- a/src/buildMathML.js
+++ b/src/buildMathML.js
@@ -145,14 +145,24 @@ export const buildExpression = function(
                     lastGroup.children.push(...group.children);
                     continue;
                 }
+            } else if (lastGroup.type === 'mi' && lastGroup.children.length === 1) {
+                const lastChild = lastGroup.children[0];
+                if (lastChild instanceof TextNode && lastChild.text === '\u0338' &&
+                    (group.type === 'mo' || group.type === 'mi' ||
+                        group.type === 'mn')) {
+                    const child = group.children[0];
+                    if (child instanceof TextNode && child.text.length > 0) {
+                        // Overlay with combining character long solidus
+                        child.text = child.text.slice(0, 1) + "\u0338" +
+                            child.text.slice(1);
+                        groups.pop();
+                    }
+                }
             }
         }
         groups.push(group);
         lastGroup = group;
     }
-
-    // TODO(kevinb): combine \\not with mrels and mords
-
     return groups;
 };
 
diff --git a/src/macros.js b/src/macros.js
index 06541df7..e8c68497 100644
--- a/src/macros.js
+++ b/src/macros.js
@@ -348,7 +348,8 @@ defineMacro("\\clap", "\\mathclap{\\textrm{#1}}");
 // \DeclareMathSymbol{\not}{\mathrel}{symbols}{"36}
 // It's thus treated like a \mathrel, but defined by a symbol that has zero
 // width but extends to the right.  We use \rlap to get that spacing.
-defineMacro("\\not", '\\mathrel{\\mathrlap\\@not}');
+// For MathML we write U+0338 here. buildMathML.js will then do the overlay.
+defineMacro("\\not", '\\html@mathml{\\mathrel{\\mathrlap\\@not}}{\\char"338}');
 
 // Negated symbols from base/fontmath.ltx:
 // \def\neq{\not=} \let\ne=\neq