We prepared thin films of magnesium nickel with a wide compositional range from pure magnesium to the intermetallic compound Mg2Ni by e-beam evaporation. Capped with a thin Pd film such layers act as switchable mirrors upon exposure to gases containing hydrogen and oxygen. We investigated the reaction kinetics by measurement of the transmittance, and the reflectances both from the film side and the substrate side. We found that the hydrogenation kinetics and the optical performance are strongly dependent on the layer composition. The best results were obtained for Mg4Ni to Mg8Ni. It is shown that this is due to the different nucleation and growth behavior of the layers. (c) 2006 Elsevier B.V. All rights reserved.