The selenisation of the thin film precursors indium, Cu-In and Cu-In-Ga has been studied by in-situ X-ray diffraction. The influence of sodium (loping on the kinetics of the solid-state reactions observed agrees with the predictions derived from a phenomenological model presented earlier. In this model a layer of sodium polyselenide is assumed to form at the surface of the precursor, or around each crystal grain. This layer might be responsible for impeded ion exchange in certain solid-state reactions. (C) 2006 Elsevier B.V. All rights reserved.