Research ethics committees (RECs) are charged with adjudicating the ethical status of research projects, and determining the conditions necessary for such projects to proceed. Both because of their position in the research process and because of the controversial nature of ethical judgements, RECs’ views and decisions need to be accountable. In this paper we use techniques of discourse analysis to show how REC decision letters ‘do’ accountability. Using a sample of 260 letters from three datasets, we identify a range of discursive devices used in letters written by RECs. These include drawing attention to: the process behind the decision, including its collaborative nature; holding the applicants accountable, by implying that any decision made by the REC can be attributed to the performance of the applicants; referring to specialist expertise; and calling upon external authorities. These tactics ‘do’ accountability by showing that routines of ethical assessment have been enacted, by establishing the factuality of claims, and by managing questions of fault and blame attribution. They may, however, also risk undermining legitimacy by failing to acknowledge the inherent contestability of ethical decision making or the limited nature of the cultural authority accorded to RECs, and thus may appear as an illegitimate exercise of power.