This study investigates the effect of comparison on the perceived similarity of unfamiliar faces. Participants were asked to compare pairs of faces, and to list either their commonalities or the differences between them before rating their similarity. The results of the first experiment show that listing commonalities and differences can both increase the perceived similarity of the compared faces. This pattern of results is interpreted in line with structural alignment theory which posits that the detection of commonalities is an essential component of both similarity and difference comparisons. The results of the second study show that the effect of comparison on perceptual similarity is limited by the perceptual attributes of the compared faces. Under conditions where the compared faces are highly similar and the detection of commonalities do not require any mental manipulation, comparison has little or no effect on perceived similarity.