In their paper, Martinez, Solanto, and Jiminez compared a number of methodologies used to describe students' understandings of scientific conceptions. One of the issues raised by the authors was the lack of a theoretical platform based in the area of cognition upon which the data were analysed. This paper investigates students' understandings of diffusion through the application of a cognitive structural perspective provided by the Structure of the Observed Learning Outcome model devised by Biggs and Collis. In this study, 60 senior secondary school and 120 first-year university science students were presented with two extended response questions regarding diffusion. Four months after the completion of the questionnaires, 30 students were interviewed. The responses obtained from the students were interpreted using the Structure of the Observed Learning Outcome model. The results from the study provided strong evidence of a pathway of conceptual understanding of diffusion from simple intuitive ideas about movement to highly abstract views in which students explained the random motion of molecules in terms of kinetic theory. These results were consistent for both the high school and university students. In addition, the pathway provided a means of interpreting previous research results and practical ways of improving instruction in the future. (Contains 3 figures and 6 tables.)