This study evaluated the effects of a cross-age peer learning program targeting vocabulary and comprehension in kindergarten and fourth-grade classrooms with substantial proportions of English Learners (ELs). The study followed a quasi-experimental design with 12 classrooms (6 kindergarten and 6 fourth grade) in the intervention group and 12 classrooms (6 kindergarten and 6 fourth grade) in the comparison group. Students were assessed before and after the 14-week intervention via curriculum-aligned and norm-referenced vocabulary and comprehension assessments. Findings of analyses of researcher-developed measures showed positive and significant intervention effects on receptive and expressive vocabulary in kindergarten and fourth grade and comprehension (i.e., understanding of text and strategy use) in fourth grade. Findings of analyses of norm-referenced measures showed positive and significant intervention effects on receptive vocabulary in kindergarten. In general, the intervention had similar effects for ELs and non-ELs.