Within dual language education programs, well-structured peer interactions can facilitate the learning of language and curricular content simultaneously. Research has found that even very young bilingual students can engage in peer linguistic mediation to help less proficient classmates participate in small group work. In this article, I analyze two academic discussions among emergent bilingual first graders in a Spanish-medium dual language classroom. This analysis demonstrates that, contrary to previous research, the language used by students did not facilitate learning. Rather, it foreclosed the possibility of effective communication across languages. Findings suggest that peer linguistic mediation is influenced by multiple factors and cannot be taken for granted. Questions are raised about the ways in which emergent bilingual students are grouped and the skills they need to successfully mediate their peers' learning. Implications from these findings go beyond dual language education to apply to mainstream classrooms with minority language speakers in them.