Recent developments in the philosophy of science and cognitive psychology suggest that students' abilities to use science processes effectively may be greatly influenced by their knowledge of relevant concepts. The central objective of this study was to determine the extent to which students' knowledge of concepts relevant to mineral classification influenced their ability to classify minerals. A second objective was to determine which concepts are most influential. Thirty-five eighth grade earth science students were tested regarding their ability to classify minerals correctly (the dependent variable) and their knowledge of mineral hardness, cleavage, luster, streak, and the logic of the classification scheme (the independent variables). This was done following their normal classroom instruction. The results of the data analysis using an all-possible regressions procedure showed that 58.7% of the variance in correct mineral classifications was accounted for by students' knowledge of relevant concepts. Mineral hardness and cleavage were particularly important. These results suggest that students' knowledge of relevant concepts substantially influences their ability to perform the science process of mineral classification. In particular, mineral hardness and cleavage should be emphasized during instruction. The results also suggest that the presently used conception of science processes as being independent of conceptual knowledge needs to be examined and researched further.