Ascertaining the thinking of professionals as they are confronted with ethical practice issues is a prerequisite to understanding ethical decision making. Before researchers or educators can examine the effectiveness of various approaches to ethics teaching and learning, there is a need for reliable and valid tools to assess practitioners' cognition. A potential problem with the few measuring instruments currently available is the fact that they ask subjects to rank order existing lists of issues. This says little about an individuals' own thinking about ethical issues and may prompt thinking or responses which would not otherwise have occurred. This paper reports the results of a study to test the psychometric properties of a new instrument, the Ethical Reasoning Tool (ERT) that measures ethical reasoning of nurses. The ERT demonstrates a promising way to reveal unprompted ethical thinking about a practice dilemma, thereby clarifying ‘real’ versus ‘assumed’ professional reasoning. The tool allows nurse educators to identify areas of student learning/reasoning deficiency that can be addressed by educational interventions. The ERT also allows nurse educators to evaluate the effectiveness of nursing ethics study units in a trustworthy way.