A 50 nm samarium film capped with a 7 nm palladium overlayer switched from a metallic to semiconducting state during ex-situ hydrogen loading via electrochemical means at room temperature. The transition is accompanied by a change in transmittance measured during hydrogen loading and the associated optical appearance. The monitoring of working electrode (WE) potential, the transmittance and chi potential difference (Deltachi) has been used to identify the phases present during hydrogen loading. Deloading of hydrogen has been studied in open circuit potential condition. Glancing angle X-ray diffraction (GAXRD) studies show that the rhombohedral structure of metallic samarium film (a(0) = 8.989 Angstrom) changes to hexagonal structure of the SmH3-delta film with average lattice parameters of a = 3.775 Angstrom and c = 6.743 Angstrom. A direct optical band gap of 2.9 eV has been obtained for SmH3-delta film and 2.0 eV for SmH2+/-epsilon film from reflectance and transmittance data. Removal of hydrogen from SmH3-delta leads to the formation of localized states within the band whose signature is clearly seen in transmittance and Tauc's plot curves of SmH2+/-epsilon, film. The Hall coefficient RH measured as a function of hydrogen concentration, changes from a metal-like value -14.23 x 10(-10) m(3)/C to -1001.1 x 10(-10 m3)/C for SmH3-delta films. On unloading hydrogen, the value of R-14 changes to -3.56 x 10(-10) m(3)/C at the dihydride composition. (C) 2004 Elsevier Ltd. All rights reserved.