A 55-nm samarium film capped with a 15-nm palladium overlayer switched from a metallic reflecting to a semiconducting, transparent in visible state during ex-situ hydrogen loading via electrochemical means in 1 M KOH electrolytic aqueous solution at room temperature. The optical transmittance exhibits a hysteresis effects on loading and unloading of hydrogen in SmHx, thin film between x=2 and 3. The hysteresis is discussed in terms of strain (and consequently stress) at the interface between the dihydride and trihydride. The diffusion coefficients and switching kinetics are shown to depend on applied current density. The changes of anodic overpotential with time during applied constant current discharge have been used to determine the electrochemical parameters the exchange current density i(o) and the symmetry factor beta. (c) 2005 Elsevier B.V. All rights reserved.