A theoretical model has been developed to explain the movement of ap‐njunction in PbSe by the interdiffusion process. This model, which is a modification of an interdiffusion theory due to Brebrick, has one value for the diffusion coefficient inp‐type material and another value inn‐type material. Solutions to the diffusion equation for this model are obtained and compared to experimental interdiffusion data for PbSe at 400°C obtained by thep‐n junction method. The significance of the results to device fabrication and annealing is discussed.