There is an increasing interest in the evolution of open-source software (OS). Researchers as well as practitioners are trying to better understand factors that impact the diffusion of OS. This paper presents an agent-based model of OS diffusion. Specifically, we investigate how software upgrade cycle affects firms’ OS adoption. In addition, we also incorporate factors such as variability in OS support costs, interoperability issues and network structure that have not been systematically studied in prior OS research. Simulation results demonstrate the individual and interaction effects of these variables on the rate of OS diffusion. High variability in OS support costs and more frequent introduction of major upgrades encourage OS diffusion. The rate of diffusion is also influenced by the degree of cliquishness in the network structure. Interoperability issues hinder OS diffusion when proprietary software (PS) is currently the dominant standard. However, if other factors encourage OS diffusion and a critical mass of OS adopters is reached, then interoperability issues encourage OS diffusion. The impact of interactions between network structures and other factors on diffusion dynamics is also illustrated.