A model for the precipitation of arsenic in arsenosilicate glass (AsSG) is developed to simulate the increased arsenic concentration at the glass/silicon interface after high temperature heat-treatment. In this model, the precipitation occurs at the interface when arsenic concentration exceeds the solubility limit and grows laterally until it covers the entire interface. This assumption is partly supported by secondary ion mass spectrometry and transmission electron microscopy results showing the presence of silicon arsenide precipitates near the interface. Reasonable agreement with published experimental data is achieved. This model can be used to predict the precipitation, and thereby develop novel techniques to avoid precipitate formation, and to optimize the material developments.