Thermodynamics of hydride formation and decomposition in electrodeposited Pd-rich (>90 at.% Pd in the bulk) Pd–Ru alloys was studied in acidic solutions (0.5 M H2SO4) using chronoamperometry. It has been found that for the temperature range 283-328 K the potentials of the α→β and β→α phase transitions are shifted negatively with both increasing temperature and decreasing Pd content in the alloy bulk. Thermodynamic parameters (Gibbs energy, enthalpy and entropy) of the β-phase formation and decomposition were determined. With decreasing Pd bulk content the process of the β-phase formation becomes less exothermic, entropy rises and thermodynamic stability of the β-phase decreases. The heats of hydride formation in Pd-Ru alloys decrease almost linearly with the values of Fermi energies.