The process of hydrogen electrosorption in thin Pd-Pt-Rh alloys has been investigated mainly in acidic solution using cyclic voltammetry and chronoamperometry. The influence of electrode potential on the amount of electrosorbed hydrogen is presented. On the plot of H/M (hydrogen atoms/metal atoms) vs. potential the regions of pure alpha-and beta-phase existence as well as the region of alpha-beta phase transition can be distinguished. The maximum amount of absorbed hydrogen depends on the bulk composition being relatively high for Pd-rich alloys, although smaller than for pure Pd. The alpha-beta transition is shifted to lower potentials in comparison with pure Pd. The process of phase transition seems to control the rate of hydrogen absorption and desorption into/from thin Pd-based electrodes. The hysteresis effect is observed in chronoamperometric absorption and desorption experiments. Potential cycling of Pd-Pt-Rh alloys through the oxygen region leads to significant changes in hydrogen absorption properties with the appearance of an additional bulk phase. (C) 2004 Elsevier B.V. All rights reserved.