Purpose: This study contributes to the science of teaching reading and vocabulary by illustrating how a ubiquitous classroom practice -- read alouds -- can be enhanced by structured supplements. This experimental study examines whether and to what extent providing structured supplements can improve student comprehension outcomes by helping teachers foster discussions about academic vocabulary that support schema transfer as students make connections between known and new topics. Method: A total of 80 third-grade teachers and their students (N = 965; 32% Black, 31% Hispanic, 25% white, 9 Asian, 48% Male) were randomly assigned to treatment or control conditions. Treatment students received a single social studies read aloud on the story of Apollo 11 with structured supplements while control students received the same read aloud story but without structured supplements. Students were from linguistically, economically, and ethnically diverse backgrounds. Results: Effect sizes from hierarchical linear models indicated that students in the treatment condition outperformed students in the control condition on four measures of domain-specific reading comprehension: recall (ES = 0.17), near-transfer (ES = 0.17), mid-transfer (ES = 0.18), and content comprehension (ES = 0.18). Further exploratory analyses using structural equation modeling revealed that teacher language scaffolds -- that is, temporary dialogic supports in which teachers went above and beyond the intervention script -- explained 66% of the treatment effect on domain-specific reading comprehension. Conclusion: Results from this study suggest that read alouds, when enhanced with structured supplements designed to facilitate schema transfer, can increase the amount of academic vocabulary teachers use during classroom instruction and improve their students' ability to comprehend disciplinary texts.