The principles underlying the perception of vowel stimuli are investigated. Chistovich (1971) claims that when judging the distance between synthetic vowel stimuli, speakers make reference to the phonemic structure of their native language as the basis of their judgements, i.e. speakers determine which phonemes are closest to the stimuli and then compare the phonemes and not the stimuli themselves. The experiment reported here tested this hypothesis. Native speakers of Danish and English judged the difference between synthetic vowel stimuli, and the judgements patterned according to the phonemic structures of the respective languages. The results support Chistovich’s hypothesis.