Universities are paying increased attention to how they might support the ethical development of their students as one of a range of graduate attributes that will enable them to negotiate increasingly complex professional, civic and personal futures. Scenario-based learning is a long-standing strategy used in ethical teaching, and this paper describes and evaluates a version of this approach as applied to a second year undergraduate tutorials module. A quantitative assessment of the development of students' ethical sensitivity over the course of two deliveries of the module shows an uneven impact but also some encouraging trends. A detailed qualitative analysis of how students responded to each scenario identifies five factors that appear to precipitate more in-depth reflection on ethical problems, and these are presented as useful points of guidance for teachers writing ethical scenarios for the first time or for those aiming to hone their existing practice. These factors include the challenge of devising circumstances which appear realistic and plausible to contemporary undergraduate students, constructing scenarios which encourage readers to reflect on and test their personal values, and portraying events which push students to intervene proactively and so take individual responsibility for their decisions and actions.