This paper presents a new constructivist model of knowledge development in a case study that illustrates how a group of postgraduate students defined and evaluated effective postgraduate supervision. This new model is based on "personal construct theory" and "repertory grid technology" which is combined with interviews and group discussion. It is argued that this approach leads to a more meaningful interpretation of results and facilitates formative evaluation and professional development of supervisors. In this case study we discuss details of our evaluation method and its benefits and limitations. We explain how this approach enables both supervisors and students to participate actively in research and development activities, to develop their own constructs or theories of effective supervision, and to communicate their suggestions for improvement. Further applications of this constructivist model to postgraduate supervision practice and to research in higher education are suggested. (Contains 2 figures and 1 table.)