This study examines whether Spanish-English bilingual children's phonological awareness (PA) performance reflects specific contrasts between English and Spanish by focusing on children's segmentation of vowels treated as single units in English but as two units in Spanish (/eI/ and /aI/). The role of oral language proficiency, specifically vocabulary, in the phonological awareness of bilingual children is also explored. Bilingual kindergartners and first graders in English or Spanish literacy instruction and a comparison group of monolingual English-speaking peers were administered tests of expressive vocabulary in English and Spanish and a phonemic segmentation task in English. Bilingual children, particularly those with higher Spanish language proficiencies, tended to hypersegment long diphthongized vowels. English language proficiency predicted correct performance on English phonemic segmentation more powerfully for children with low Spanish language proficiency and for children in English literacy instruction. For Spanish-instructed children, Spanish language proficiency predicted English phonemic segmentation, suggesting cross-language transfer. (Contains 32 references.) (Author/SM)