The promising new generation of solar cells based on CIGS (Cu(I,Ga)Se-2) exhibits behavior differing from that of earlier cells because of changes in the method of preparation, leading, among other things, to a difference in sodium content. A simple defect chemical model is presented for the effects of sodium on the surface chemistry and electronic properties of CIGS thin films. The model, based on the well-known catalytic effect that alkali metals have on surface oxidation of semiconductors, is shown to be consistent with the experimental data available in the literature.