Small particles of the semiconducting transition-metal cluster material Mo4.2Ru1.8Se8 were incorporated into a Nafion(R) (DuPont) film attached to various electrodes. Oxygen reduction in acid medium and photoevolution of hydrogen were significantly enhanced on glassy carbon and p-type GaP, respectively. However, within the Nafion layer the surface area of the catalytically active material exposed and involved in oxygen reduction was comparatively small. For the modified GaP photoelectrode, the improvement of hydrogen evolution has been interpreted in terms of a negative shift of the quasi-Fermi level caused by a bypass of surface trapping of electrons induced by adsorbed hydrogen species. Photoevolution of oxygen from water on modified n-type RuS2 did not succeed owing to the oxidation of the catalyst. It is concluded that the new material could, in the form of finely divided particles, compete with platinum in certain reduction mechanisms as a catalyst for multi-electron-transfer reactions.