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: generating one or more splits of the first compound term, 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 a first split from the one or more splits according to the overall score for each split, and augmenting the query with a first synonym phrase that is a synonym of a first subterm of the first split.