Our study proposes the use of a free classification task for investigating the dimensions used by listeners in their perception of nonnative sounds and for predicting the perceptual discriminability of nonnative contrasts. In a free classification task, participants freely group auditory stimuli based on their perceived similarity. The results can be used to predict discriminability and can be compared to various acoustic or phonological dimensions to determine the relevant cues for listeners. The viability of this method was examined for both a segmental contrast (German vowels) and a nonsegmental contrast (Finnish phonemic length). Perceptual similarity data from the free classification task accurately predicted discriminability in an oddity task and separately provided rich information on how the perceptual space is shaped. These results suggest that a free classification task and related analyses are informative and replicable methods for examining nonnative speech perception.