Methods, systems, and apparatus, including computer programs encoded on computer storage media, for identifying synonyms. One method includes receiving a query containing a first phrase, identifying one or more first synonym phrases that are synonyms for the first phrase, identifying a new synonym phrase that is a synonym for one of the first synonym phrases, determining that the new phrase is a synonym for the first phrase, and augmenting the query with the new phrase. Another method includes receiving a query including a first compound term having a first subterm, identifying a first synonym for a first subterm, generating a second compound term, wherein the second compound term is the first compound term modified by replacing the first subterm with the first synonym, and augmenting the query with the second compound term.