The switchable optical properties of Pd-protected RCo2-type Ho(0.6)Mm(0.4)Co(2) alloy thin films have been investigated in a KOH electrolyte. The reversible optical switching has been carried out simultaneously by measuring transmitted light through the thin film during electrochemical charging-discharging of hydrogen. The dependence of switching speed and cyclic durability of the film on the charging and discharging current density as well as concentration of KOH electrolyte has been studied. in addition, cyclic voltammetric measurements have been performed to examine the hydride formation and decomposition reactions. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.