Methods, systems, and apparatus, including computer programs encoded on computer storage media, for evaluating substitute terms. One of the methods includes receiving a query having an original term and determining one or more substitution contexts for the original term, wherein a substitution context includes one or more context terms and an indication of a position in the query of the original term and the one or more context terms. The substitution contexts are classified into a first category or a second category based on a respective score of each substitution context. The original term is associated with one or more substitution contexts in the first category.