This qualitative study examined whether and how preschoolers' social interactions during buddy "reading" supported their use and learning of theme-related vocabulary. Data included 32 transcribed videos of 14 preschoolers engaged in buddy "reading." Interaction analysis and constant comparative methods were applied to identify (1) patterns of social interactions that occurred, (2) patterns of opportunities for children to use and learn themed vocabulary, and (3) patterns that hindered such vocabulary opportunities. Findings included three patterns of social interactions: collaborative, tutor-tutee, and parallel. Each differentially affected opportunities for using and learning theme-related vocabulary. Opportunities to use and learn words occurred when children provided exposures to vocabulary words or clarified concept knowledge for their buddy. Opportunities were hindered when buddies struggled over participation, "reading" roles, or content.