Sulphur diffusion in the CdTe layer of a CdS/CdTe solar cell has been simulated using a finite difference algorithm that assumed a columnar grain structure approximated by a cylindrical grain. It differs from previous studies by the inclusion of a 'dynamic' diffusant source, as opposed to the constant, infinite sources used in previous works. The simulations demonstrate rapid diffusion into the grain boundary as compared to the bulk of the grain. However, it is also shown that the grain-boundary diffusion may be limited by the rate at which diffusant is supplied to the grain boundary, i.e. by the mobility of S in CdS. (C) 2003 Elsevier Science B.V. All rights reserved.