We have developed a model to simulate the phase-change behavior of GeSbTe thin film alloys. Based on classical nucleation theory we described formation of crystalline clusters using chemical rate equations. Assuming that the phase-change proceeds by interactions of single GeSbTe molecules with growing or decaying crystalline clusters we used a set of differential equations to account for the population density changes of clusters. We defined reaction rates encountered in model equations by considering possible molecular processes during the phase-change process. To validate the model we simulated experiments taken from the literature. It can predict the kinetics of crystallization well, describe transient effectis correctly, and consider influecnes of substrates on crystallization successfully.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.