Thin, Pd-capped metallic films containing magnesium and first row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous hydrogen or following cathodic polarization in an alkaline electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepared by co-sputtering from one magnesium target and one manganese, iron, or cobalt target. Both the dynamic optical switching range and the speed of the transition depend on the magnesium-transition metal ratio. Infrared spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg 3 MnH 7 , Mg 2 FeH 6 , and Mg 2 CoH 5 . * Following the discovery of the switchable mirror phenomenon in yttrium and lanthanum hydrides by Huiberts et al., 1 similar behavior was reported in rare earth-magnesium alloy films. 2 Recently, metaltransparent hydride switching was also found in nickel-magnesium thin films. 3 In all of these systems, a thin Pd overlayer (generally > 5 nm) is applied to catalyze absorption and desorption of hydrogen and to protect the readily oxidized rare earth and/or Mg from oxidation. Infrared internal reflectance spectroscopy was used to characterize the transparent Ni-Mg hydride films as a mixture of Mg 2 NiH 4 and MgH 2 . 3 Other ternary hydrides containing magnesium and a first row transition metal include Mg