Quantitative education research has yet to assemble a cohesive perspective on the impact of bilingual programs on Spanish-speaking students’ achievement. The current study offers a nearly unprecedented methodological advancement in bilingual education research through a comprehensive, comparative analysis of reading achievement across four major language acquisition programs in U.S. schools—transitional, one-way developmental, two-way bilingual immersion, and English immersion. Furthermore, the study contextualizes these programs and their outcomes within segmented assimilation theory, theoretically linking each program to a specific form of acculturation. Using a longitudinal dataset from a large urban school district in Texas, I employed multilevel models to examine reading outcomes in elementary school. Results indicate that transitional and two-way students have comparable reading outcomes in Spanish and English. Across grade levels, one-way students demonstrate lower Spanish reading growth than transitional students. The English reading analysis shows that program exposure beyond four years is linked to significantly lower reading performance across all students, and among English immersion students in particular. These results indicate that transitional programs—although associated with consonant acculturation’s limited bilingual foundation—lead to similar, and in some cases higher, reading outcomes compared to fully additive bilingual programs such as one-way or two-way.