English monolinguals and native Spanish speakers of English rated the dissimilarity of tokens of two Spanish vowel categories, two English vowel categories, or one Spanish and one English vowel category. the dissimilarity ratings of experienced and inexperienced Spanish subjects did not differ significantly. for both the native Spanish and English subjects, perceived dissimilarity increased as the distance between vowels in an F1 —F2 acoustic space increased. This supported the existence of a universal, sensory-based component in cross-language vowel perception. The native English and Spanish subjects' ratings were comparable for pairs made up of vowels that were distant in an F1 —F2 space, but not for pairs made up of vowels from categories that were adjacent in an F1 —F2 space. The inference that the differential classification of a pair of vowels augments perceived dissimilarity was supported by the results of experiment 2, where subjects rated pairs of vowels and participated in an oddity discrimination task. Triads in the oddity task were made up of tokens of vowel categories that were either adjacent (e.g.,/a/-/æ/-/a/) or nonadjacent (e.g.,/a/-/i/-/i/) in an F1 —F2 space. The native English subjects' discrimination was better than the native Spanish subjects' for adjacent but not nonadjacent triads. The better the Spanish subjects performed on adjacent triads—and thus the more likely they were to have differentially classified the two phonetically distinct vowels in the triad—the more dissimilar they had earlier judged realizations of those two categories to be when presented in pairs. Results are discussed in terms of their implications for second language acquisition. © 1994, Acoustical Society of America. All rights reserved.