This study, conducted in an inner-city middle school, followed the conceptual changes shown in 25 students' writing over a 12-week science unit. Conceptual changes for 6 target students are reported. Student understanding was assessed regarding the nature of matter and physical change by paper-and-pencil pretest and posttest. The 6 target students were interviewed about the goal concepts before and after instruction. Students' writing during lesson activities provided qualitative data about their understandings of the goal concepts across the science unit. The researcher constructed concept maps from students' written statements and compared the maps across time to assess changes in the schema of core concepts, complexity, and organization as a result of instruction. Target students' changes were studied in detail to determine patterns of conceptual change. After patterns were located in target students' maps, the remaining 19 students' maps were analyzed for similar patterns. The ideas that students identified in their writing showed changes in central concepts, complexity, and organization as the lessons progressed. When instructional events were analyzed in relation to students' demonstrated ideas, understanding of the goal conceptions appeared in students' writing more often when students had opportunities to explain their new ideas orally and in writing.