Evidence suggests that verbal engagement is central to young children’s language learning. Furthermore, characteristics of engagement, such as language quantity and quality, can either hinder or encourage vocabulary growth. However, the same quantity of input results in larger vocabularies in English- relative to Spanish-dominant children. The present study seeks to assess variation in quantity and quality of communicative engagement in English- and Spanish-dominant children in the U.S. and their parents to determine the relation between these factors and observed differences in early vocabulary. Participants were 47 Spanish- and 50 English-dominant parent-child dyads who participated in a 20-minute recorded free-play session. Transcripts of the play session provided the parent’s total words used (quantity) and were coded for quality considerations which were divided into two categories: parent-child engagement and parent responsiveness. Parents completed the MCDI, an established measure of child expressive vocabulary. Analyses were conducted separately, but in parallel, for the two categories of quality. Initially, we conducted linear mixed-effects analyses predicting vocabulary to produce Akaike Information Criterion. The models with best fit and parsimony included language and engagement/responsiveness variables, but no interactions. Multiple regression analyses were then performed on the final models predicting child vocabulary to obtain R2 and standardized beta-weight estimates. The engagement model accounted for 57% of the variance in child vocabulary across languages. Language and two patterns of engagement were significant predictors of child vocabulary. The parent responsiveness model accounted for 47% of the variance in child vocabulary across languages. Language exposure and two patterns of parent responsiveness were significant predicters of vocabulary. However, in this model, the variance contributed by language was fully subsumed by the parent responsiveness variables. In this sample of 30-month-old Spanish-and English-speaking toddlers, we found that parent-child engagement and parent responsiveness account for significant variance in child expressive vocabulary and this pattern holds across Spanish and English speakers. Importantly, parent responsiveness accounted for the gap in vocabulary size between languages such that language was no longer a significant predictor in that model whereas parent-child engagement functioned similarly in both languages studied, which suggests that these may be universal characteristics of high-quality parent-child engagement.