The processes of hydrogen absorption/desorption into/from Pd and Pd alloys with Pt and Rh have been studied using the electrochemical quartz crystal microbalance (EQCM). The frequency shift connected with hydrogen electrosorption exceeds the value predicted by Sauerbrey equation and can be attributed not only to the mass changes but also to the stresses generated inside the metal/alloy. The influence of stresses on the EQCM response depends oil the amount of electrosorbed hydrogen and the kind of the system (pure Pd or an alloy of a given composition) as well as being sensitive to the effect of electrode ageing. The magnitude of stresses measured by EQCM correlates with the relative change in the lattice constant caused by hydrogen absorption. EQCM experiments indicate that the magnitude and distribution of stresses are different for absorption and desorption processes, which seems to confirm the important role of the stress effect in the phenomenon of absorption/desorption hysteresis. (c) 2006 Elsevier B.V. All rights reserved.