Drawing on an interview-based study of the beliefs, practices, and experiences of 16 proofreaders of student writing at undergraduate and/or graduate level in a university setting, this paper focuses on the ethical concerns informants associate with the proofreading act. "Proofreading" is defined for the purposes of this research as "third-party interventions (that entail some level of written alteration) on assessed work in progress". Informants' ethical concerns related, broadly, to: (i) the text itself and the nature of interventions; (ii) the roles and relationships of proofreaders, writers, and their lecturers/supervisors, and how these impact on each other; and (iii) the wider university context. Text types identified by some informants as ethically problematic to correct were those by low proficiency writers and those of poor quality in terms of subject knowledge. Both corrections and comments were used by informants to draw attention to problems in writers' texts in an ethical manner. It was reported that some writers have inappropriate expectations of proofreaders, expecting help which informants regard as ethically indefensible. While some informants reported that they did not experience ethical dilemmas about appropriate levels of intervention, others reported much uncertainty, and called for more explicit guidance from university authorities. The implications of the study are discussed.