L2 student writers in UK universities often seek the services of a 'proofreader' before submitting work for assessment, and the proliferation of freelance proofreaders and online proofreading agencies has led to debates about the ethics of the proofreading of student writing in publications such as "Times Higher Education". This study investigates the kinds of ethical issues that confront proofreaders and how they deal with them. Nine UK university proofreaders proofread the same poor-quality L2 applied linguistics master's essay, explaining their interventions by talking aloud while proofreading and at a subsequent interview. Proofreaders addressed ethical difficulties by means of two macrostrategies: (i) selective proofreading; and (ii) declining to proofread in part or in whole. Two additional findings relating to ethics emerged from the study: firstly, some informants experienced dilemmas and uncertainties despite their attempts to proofread ethically; secondly, a number of informants went far beyond traditional, narrow conceptualizations of proofreading, making interventions affecting the writer's structure, argumentation, and content which could be seen as unethical. The findings highlight the need for the regulation of proofreading to ensure it is standardized and consistently administered from writer to writer, and I close by recommending that universities strive to implement more formative types of proofreading to enhance writers' academic literacy, not just their texts.