This study examined the representational competence of students as they solved problems dealing with the temperature-pressure relationship for ideal gases. Seven students enrolled in a first-semester general chemistry course and two advanced undergraduate science majors participated in the study. The written work and transcripts from videotaped think-aloud sessions were evaluated with a rubric designed to identify essential features of representational competence, as well as differences in student use of multiple representations. The data showed that both beginning and advanced chemistry students tend to prefer one type of representation. However, advanced students were more likely to use their preferred representations in a heuristic manner to establish meaning for other representations. Students were found to build conceptual understanding most easily when using familiar types of representations. Molecular-level sketches representing dynamic concepts not easily represented as static images, such as an increase in average molecular velocity, were the most difficult type of representation for students to interpret. These results suggest that students may benefit from instructional strategies that emphasize the heuristic use of multiple representations in chemistry problem solving. (Contains 4 tables and 3 figures.)