Methods, systems, and apparatus, including computer programs encoded on computer storage media, for index-side synonym expansion. One method includes obtaining a token sequence for a resource and indexing a particular token in the token sequence. The indexing includes obtaining a diacritically canonicalized form of the particular token; determining that the diacritically canonicalized form of the particular token is different from the particular token; and storing data associating the resource with both the particular token and the different diacritically canonicalized form of the particular token as index terms for the resource in a search engine.