The process of students' conceptual change was investigated during a computer-supported physics unit in a Grade 10 science class. Computer simulation programs were developed to confront students' alternative conceptions in mechanics. A conceptual test was administered as a pre-, post-, and delayed posttest to determine students' conceptual change. Students worked collaboratively in pairs on the programs carrying out predict–observe–explain tasks according to worksheets. While the pairs worked on the tasks, their conversational interactions were recorded. A range of other data was collected at various junctures during instruction. At each juncture, the data for each of 12 students were analyzed to provide a conceptual snapshot at that juncture. All the conceptual snapshots together provided a delineation of the students' conceptual development. It was found that many students vacillated between alternative and scientific conceptions from one context to another during instruction, i.e., their conceptual change was context dependent and unstable. The few students who achieved context independent and stable conceptual change appeared to be able to perceive the commonalities and accept the generality of scientific conceptions across contexts. These findings led to a pattern of conceptual change which has implications for instructional practices. The article concludes with consequent implications for classsrooms. © 1999 John Wiley & Sons, Inc. J Res Sci Teach 36: 859–882, 1999