Categorization is a fundamental psychological ability necessary for problem solving and many other higher-level cognitive tasks. In organic chemistry, students must establish groupings of different chemical compounds in order not only to solve problems, but also to understand course content. Classic models of categorization emphasize similarity as the primary means of classification: if two items are similar, they belong in the same category and if they are dissimilar, they belong in different categories. These models also underscore a one-dimensional approach where a single salient feature is often used as the basis of categorization, even though multiple salient features may be present. This study examined the critical attribute organic chemistry students used to categorize eight organic compounds possessing similar salient features related to structure, functional groups, and stereochemistry and how this primary feature changed over time. Results from this study support the notion that the critical attribute is not stable, but changes as student learning progresses. The results also suggest that categorization is a systematic process involving conscious selection of the critical attribute and that higher-ability students (e.g., honor students) are more prone to be influenced by the current instructional topic when selecting the critical attribute. (Contains 3 figures.)