I am trying to add a particular word to my custom dictionary using Word 2007. (The word happens to be "deserialized".) When I right-click on the word and click Add to Dictionary, the red underline does not go away. When I use the Spelling & Grammar tool from the Review tab on the ribbon, it will stop on that word; clicking the Add to Dictionary button has no effect. Oddly, I am able to add other words to the custom dictionary without a problem. I recently added "deserializes", for instance. I have only encountered this problem with that one particular word. Does anyone know what might be wrong and how I might fix it? Clarifications My document and all its content is set to English (United States). My custom dictionary is set to apply to All Languages, which is the default value. "Serialize" is in the US English default dictionary, but "deserialize" and its various forms is not.