The objective of this paper is to investigate, through a qualitative study based on 38 semi-structured interviews with agents who provide assurance of sustainability reports, how they perceive and manage ethical issues underlying the verification of sustainability reports. Most of the ethical issues observed involve four interconnected aspects: the commercialism underlying sustainability assurance, the symbolic nature of the verification process, interdependency between auditing and consulting activities, and familiarity with the audited companies. The findings shed light on the reflexivity of assurance (...) providers on these issues and the legitimation strategies used to explain how they reconcile the independence and impartiality required for auditing activities with commercial aspects related to client–provider relationships. The study also shows the role of contextual variables in the ethics of assurance services. The paper contributes to the literature on the legitimacy of sustainability assurance and commercialism of the audit function. Practical implications and avenues for future research are also developed. (shrink)