Two experiments were run to investigate language-specific and universal acoustic-perceptual characteristics of the /i/–/e/ and /u/–/o/ contrasts in two languages that differ widely in the sizes of their vowel inventories, namely, English and Spanish. In order to investigate the relationship between production and perception of these two vowel pairs, the first experiment compared the acoustic and perceptual characteristics of the /i/–/e/ and /u/–/o/ contrasts within each of the two languages. In order to investigate the responses of listeners to non-native stimuli, the second experiment presented native listeners of English and Spanish with synthetic /i/–/e/ and /u/–/o/ stimuli that were modeled after natural vowels as spoken by speakers of the other language. The results indicated a close production-perception link, as well as a degree of flexibility of the perceptual vowel space in response to stimuli that are structured around non-native vowel category locations. These results suggest that listeners perceive vowel stimuli with reference to a specific phonemic vowel system, and, if necessary, will adjust their perceptual vowel system to match the stimuli. Additionally, a comparison of the absolute locations of the /i/–/e/ and /u/–/o/ perceptual boundaries across the two languages indicated the presence of universally preferred, inventory-independent bou