The purpose of the study was to explore how features of parent-child extra-textual talk during information book-sharing might vary across different socioeconomic backgrounds, and to determine if certain interactional patterns might mediate their effects on children's receptive and expressive vocabulary development. Sixty parents and their 5-year-old children were audio-recorded reading an unfamiliar information book in their home. Holistic coding on six parent and two child engagement scales, examining low to high cognitive demanding talk was conducted. Results indicated that lexical richness and contingent responsiveness positively predicted receptive and expressive vocabulary. Further, contingent responsiveness appeared to mediate the influence of socioeconomic status on children's receptive and expressive vocabulary, suggesting that positive environmental contexts and supportive parent-child interactions can have a powerful influence on children's development.