A comparative study of hydrogen and deuterium sorption into thin layer palladium electrodes from potassium and caesium hydroxide (or deuteroxide) solutions has been performed. The cations of the supporting electrolyte have a different influence on electrosorption and oxidation of hydrogen (deuterium) in or from palladium. All cations seem to be incorporated into palladium during absorption and they have a significant influence on the alpha-to-beta phase transition. The results obtained are compared with those of solutions containing lithium and sodium cations (A. Czerwinski and R. Marassi, J. Electroanal. Chem., 322 (1992) 373; A. Czerwinski, Electrochim. Acta, 39 (1994) 431).