Previous studies revealing that monolingual and bilingual infants learn similar sounding words with comparable success are largely based on prior investigations involving single-feature changes in the onset consonant of a word. There have been no investigations of bilingual infants' abilities to learn similar sounding words differentiated by vowels. In the current study, 18-month-old bilingual and monolingual infants (n = 90) were compared on their sensitivity to a vowel change when learning the meanings of words. Bilingual infants learned similar sounding words differing by a vowel contrast, whereas monolingual English- and Mandarin-learning infants did not. Findings are discussed in terms of early constraints on novel word learning in bilingual and monolingual infants.