The electrolytic insertion of hydrogen into Pd79.5Rh20.5 (at.%) alloy from acid and alkaline electrolytes was investigated in the temperature range 25-95 degreesC. It was found that, at 25 degreesC, the alloy can be loaded up to hydrogen to metal (H/M) ratios (0.90 in acid, 0.95 in alkaline electrolytes) comparable to those achieved by H-2 pressures above 10(8) Pa. The desorption isotherm of beta hydride obtained in 0.5 M H2SO4 at 25 degreesC and the thermal decomposition of this phase do not differ greatly from the expectations of the thermodynamics of the Pd80Rh20 system. At temperatures above 25 degreesC, high H/M ratios (from 0.95 to 0.93) and the unpredicted thermal stability of the P hydride phase were found only after cathodic loading in alkaline electrolytes. The phenomenon is explained by co-deposition of hydrogen and alkali metal on the alloy, which partially hinders the exit of loaded hydrogen. (C) 2003 Elsevier Ltd. All rights reserved.