A model of expertise in chemistry problem solving was tested on undergraduate science majors enrolled in a chemistry course. The model was based on Anderson's "Adaptive Control of Thought-Rational" (ACT-R) theory. The model shows how conceptualization, self-efficacy, and strategy interact and contribute to the successful solution of quantitative, well-defined chemistry problems in the areas of stoichiometry, thermochemistry, and properties of solutions. A statistical path analysis and students' explanations supported the model and indicated that the students' problem conceptualization and chemistry self-efficacy influenced their strategy use, which, in turn, strongly influenced their problem-solving success. The implication of these findings for future research and developing students' expertise in chemistry problem solving is that a strategy is advantageous when it is built on a foundation of conceptual knowledge and chemistry self-efficacy. (Contains 5 tables and 3 figures.)