We have determined the activation energies of sodium diffusion from the soda-lime glass substrate through the Mo back-contact layer, as well as through copper indium gallium diselenide (CIGS) deposited on the Mo back-contact layer of CIGS thin-film solar cells. The activation energies were determined by X-ray photoelectron spectroscopy (XPS) to measure surface sodium concentrations before and after thermally induced diffusion. The activation energies were found to be similar for the diffusion of Na through the Mo/glass and CIGS/Mo/glass thin films, approximately 8·6 and 9·6 kcal/mol, respectively. Furthermore, the sodium diffusion was found to occur by annealing in an environment of 1·0×10−5 Torr of air, oxygen, or water vapor, but not in vacuum of less than 1×10−8 Torr. In addition, the diffusion of Na was found to occur faster in the presence of oxygen than in water under identical annealing conditions. Copyright © 2003 John Wiley & Sons, Ltd.