This paper reports findings on the changes in students' understandings of the concept of matter during an academic year, for students from grade 3 through high school chemistry. The instrument for measuring students' understandings of matter consists of three forms: one for grades 3-6, one for grades 7-9, and one for grades 10-12. The three forms are linked by common items. Students' scores on the three forms are converted into Rasch scale scores so that direct comparison among students of different grades is possible. The findings show that students' understandings of four aspects of the concept of matter--structure and composition of matter, chemical properties and change, physical properties and change, and conservation of matter--are interrelated; students develop their understandings of matter gradually from elementary through high school with no apparent spurt, although their understanding does not change significantly during an academic year. (Contains 3 tables and 1 figure.)