Methods, systems, and apparatus, including computer programs encoded on computer storage media, for query synonym expansion. One method includes receiving a query including a first compound term, and in response to receiving the query, performing the following operations before search results responsive to the query are identified: generating one or more splits of the first compound term, wherein each split divides the compound term into two or more subterms, assigning a score to each subterm of each split, determining an overall score for each split from the scores for the subterms of the split, selecting one or more of the one or more splits according to the overall score for each split, and augmenting the query with the subterms of each selected split.