Writing is a critical dimension of literacy that is grounded in language and intimately connected to reading. However, instruction to support writing remains understudied, particularly among bilingual students. The purpose of this study was to examine the effects of the CLAVES intervention specifically on argument writing. The CLAVES intervention is a multicomponent, small-group intervention focused on language and literacy that was designed with bilingual learners in mind. CLAVES is an acronym for comprehension, linguistic awareness, and vocabulary in English and Spanish and means keys or clues in Spanish. It includes text-based discussions, explicit language instruction, and support for argument writing. The study was conducted with 239 Spanish-English or Portuguese-English students in grades 4 and 5. There were 120 students in the intervention and 119 students in the business as usual control group. Findings showed positive effects of the intervention on two identified aspects of argument writing: argument and counterargument.