This paper reports on the development of a research instrument designed to explore ethical reasoning in a tax context. This research instrument is a version of the Defining Issues Test (DIT) originally developed by Rest [1979a, Development in Judging Moral Issues (University of Minnesota Press, Minneapolis, MN); 1979b, Defining Issues Test (University of Minnesota Press, Minneapolis, MN)], but adapted to focus specifically on the environment encountered by tax practitioners. The paper explores reasons for developing a context- (and profession-) specific test, and details the manner in which this was undertaken. The study on which it is based aims to compare the reasoning of tax practitioners in the tax-specific context and in the general social context covered by the original DIT, and to compare this with the reasoning of non-specialists in these two contexts. The paper therefore also considers the issues that arise when using such tests to compare reasoning in different domains or to compare groups. The focus on instrument development to measure ethical reasoning in a specific domain will contribute to the literature on research methods in the area of the DIT and will facilitate cross-study comparisons.