A new model for copper sulfidation by gaseous H2S is described. The analysis focuses on the transport of charged lattice defects in a growing Cu2S product layer between the ambient gas and the substrate metal. This transport is postulated to occur via both diffusion and electromigration, and chemical reactions at the two phase interfaces serve as sources and sinks for the defect species. The pseudo-steady, one-dimensional governing equations for this system are solved by both analytical and numerical techniques, giving the instantaneous sulfidation rate as a function of the H2S concentration and the current sulfide layer thickness. It is shown that the model can reproduce quite well the results of some recent laboratory experiments in a stirred reactor. Furthermore, the analysis suggests that an assumption of electroneutrality in the sulfide, which can drastically simplify the computations, gives rise to very little error under typical conditions.