Abstract
Learning to read in a second language (L2) poses unique challenges for emergent bilingual (EB) children. This study examines whether Spanish-English EB children's vocabulary skills in Spanish and English mediate the relationship between their language input-output and English (L2) reading skills. The sample included 338 Latina/o EB children in first and third grades enrolled in dual-language immersion (DLI) programs in the United States. Results from the multivariate regression model revealed differences in language input-output at home, with third graders reported using more English than first graders. However, no grade-level differences were observed in language input-output at school in both languages, underscoring the important role of DLI programs in maintaining EB children's first language (L1). The findings also revealed the robust contributions of both L1 and L2 input-output to English vocabulary, which was positively associated with advanced English reading outcomes. When examining the indirect effects, EB children's English use at home and Spanish use at school significantly predicted their English reading outcomes through their advanced English vocabulary skills. These findings demonstrated the additive role of home language use in supporting L2 reading development. The results also underscore the importance of fostering EBs' vocabulary in both languages to enhance their English reading.