Language development of preschoolers can be positively stimulated and enhanced by exposure to language of peers. Likewise, interactions of young learners with native-monolingual classmates may contribute to their L2 development. However, research on peer interaction effects in young learners' domains of L2 development is meager. This qualitative study investigated incidental vocabulary learning of an immigrant emergent bilingual child through his interactions with monolingual classmates in a preschool classroom. The focal child was observed and videotaped across preschool year. Videotapes of 33 peer interaction episodes were analyzed with discourse analytic approach to document the patterns and processes of making vocabulary gains from peers. Results showed that interactions with monolingual peers contributed to vocabulary learning of the focal child and enriched his expressive word repertoire. Incidental peer-initiated vocabulary scaffolds provided language-mediating patterns that resulted in vocabulary gains. Conditions that facilitated or impeded vocabulary learning from peers, as well as gradual improvement in peer interaction skills, were documented. Informed by sociocultural theory and L2 expert-novice notion, this study emphasizes the significance of peer interactions to promote vocabulary learning of young children. Broadly interpreted, results suggest increased exposure to language of monolingual peers mediate L2 development of emergent bilingual children in early childhood classrooms.