We examined the extent to which positive interactions with peers and the amount of English exposure received from them during social interactions in the fall of preschool contributed to low-income Spanish-speaking children's (N=107; Mage=53 months; SD=4.30 months; 56% boys) English vocabulary and letter-word skills in the spring (controlling for parents’ language use, family income, number of English books at home, and children's nonverbal cognitive abilities). We also examined the mediating roles of children's learning behaviors (e.g., attentiveness, independence, initiative, persistence, and participation) and English oral proficiency in the classroom. The association between positive peer interactions and English vocabulary skills was mediated by children's English oral proficiency, whereas the association between positive peer interactions and English letter-word skills was mediated by children's learning behaviors and English oral proficiency. The associations among peer English exposure and children's English vocabulary and letter-word skills were mediated by children's English oral proficiency. There was also evidence of a transactional association between positive peer interactions and children's learning behaviors and between peers’ and children's English oral proficiency. The findings highlight the importance of peer experiences in fostering Spanish-speaking preschoolers’ English vocabulary and letter-word skills.