This longitudinal study investigated the progression in junior high school (JHS) students' conceptions of the structure of matter while studying a new instructional approach dealing with "Materials." In particular, we studied the progression of students' learning along two dimensions: (a) the conceptual model; and (b) the context of application. Students were asked to draw the structure of several materials and to write their explanations about the structure of these materials in questionnaires administered five times during a 3-year period. Results indicate students' progression in their microscopic conceptualization of materials. Toward the end of the instruction about 85% of the students used a microscopic model in their representations, and 36% were able to give a molecular model. About 83% of the students retained a microscopic model. Different profiles of JHS students' progression in the conception of the structure of matter were identified. The study suggests that a long-term development of the particulate model requires: (a) constructing a solid foundation of knowledge about microscopic structure of materials; and (b) a spiral instruction. (Contains 4 tables and 6 figures.)