This article reports on a formative experiment involving the implementation of long-term multifaceted vocabulary instruction in second-grade Spanish-English dual-immersion classes. The findings indicate that students in the second year of the project showed accelerated growth in English vocabulary knowledge in comparison to the norming sample of a standardized vocabulary test, and a comparison of gain scores from Year 1 (minimal treatment) and Year 2 (full treatment) on this measure further demonstrated the benefits of the full treatment condition. In addition, although the Latinx emergent bilingual students scored considerably lower than the native English-speaking students, the emergent bilinguals showed accelerated growth of nearly a half of a standard deviation from fall to spring in comparison to the native English speakers. The authors conclude by discussing the significance of the findings, particularly given the widely recognized and urgent need for programs that will accelerate the English language development of emergent bilinguals.