Metadiscourse has been conceptualized as a means to organize discourse, convey interpersonal and evaluative meanings, as well as engage the reader or listener. Importantly, metadiscourse has been theorized to uncover thought mediation during the essay-composing process. This study compares the metadiscourse in the reflective essays of 180 fifth graders, who either participated in small-group discussions using an approach called Collaborative Reasoning (CR), or who did not. Comparative analysis involving six major categories and forty subcategories of metadiscourse revealed, among other findings, that CR-exposed writers better signaled illocutionary force of reasoning, made greater use of engagement imperatives/directives and common-good rather than self-centered attitude marking. CR writers organized their ideas in a more argument-befitting logical-temporal non-list-like structure. Control students made greater use of emphatics, more often introduced hypothetical scenarios, and more frequently linked propositions together with simple additive conjunctions. The findings suggest that CR students have greater concern for how readers will take their arguments and greater appropriation of argument-enhancing formal elements, thus revealing cross-modal transfer from oral to written discourse.