Experimental data are presented for the diffusion of arsenic in polycrystalline silicon (polysilicon) during a 15‐min anneal at 900 °C. A new model is used to describe the data, which takes account of the different dopant diffusion rates in grains and in grain boundaries. The model is divided into early and late stages and, in particular, the early stage model identifies grain growth as the major mechanism by which arsenic transfers from the grains to the grain boundaries. In the late stage model, grain growth can be ignored and analytic solutions for the arsenic concentration are derived.