Diffusion and osmosis are two concepts that are fundamental to a study of biology at both a secondary and tertiary level. Both these concepts cause difficulties for students as demonstrated by the alternative conceptions held by students identified in numerous documents in the research literature (Okeke & Wood-Robinson, 1980; Westbrook, 1987; Odom, 1992; Zuckerman, 1993). A critical aspect of the understanding of these processes is their abstract nature and it appears that it is this feature which lies at the root of the problems faced by students. However, the nature of the development of these abstract ideas has not been chartered, nor has a comparison of the development for each concept. This paper offers an initial attempt to both discern, and compare and contrast the developmental growth of these concepts with 60 year 11 and 12 students. The SOLO Taxonomy was used as the evaluative tool which provided the theoretical basis to explore the answers to a series of open-ended questions. The results offer insights into the nature of formal thinking associated with the two concepts and how students' earlier understandings provide the basis for this development.