Are well-designed computer simulations an effective tool to support student understanding of complex concepts in chemistry when integrated into high school science classrooms? We investigated scaling up the use of a sequence of simulations of kinetic molecular theory and associated topics of diffusion, gas laws, and phase change, which we designed and experimentally tested. In the two effectiveness studies reported, one in a rural and the other in an urban context, chemistry teachers implemented two alternate versions of a curricular unit--an experimental version, incorporating simulations, and a control version, using text-based materials covering the same content. Participants were 718 high school students (357 rural and 361 urban), in a total of 25 classrooms. The implementation of the simulations was explored using criteria associated with fidelity of implementation (FOI). Each context provided insights into the role of FOI in affecting the effectiveness of the interventions when working with groups of teachers. Results supported the effectiveness of this sequence of simulations as a teaching tool in a classroom context, and confirmed the importance of FOI factors such as adherence and exposure in determining the specific environments in which these materials were most effective. (Contains 4 figures and 4 tables.)