Children in early childhood education and care (ECEC) spend a considerable amount of time interacting with their peers. However, open questions remain on whether and how children influence their peers' language development. The present study examined effects of peers' German receptive vocabulary (n = 1,871) on individual children's (n = 431) receptive vocabulary gains. Target children were between 30 and 48 months old at the beginning of the study. Findings revealed no links between peers' vocabulary skills and individual children's vocabulary gains, neither for all children nor depending on children's prior vocabulary skills. However, for children with lower prior skills, there was a negative association between the percentage of dual language learners in the classroom and children's vocabulary gains. This link was not attributable to peers' vocabulary skills. A number of possible mediators and moderators for peer effects were examined but none of them proved to be significant. In sum, these (null-) findings inform future research on compositional and peer effects as well as their underlying processes. Moreover, they have important practical implications for policymakers and ECEC professionals.