Powder and thin layers (<0.5 mu m) of transition metal chalcogenide deposited on various substrates (glass, gc, ito) were synthesized at 140 degrees C. The catalyst compound was obtained when reacting the corresponding transition metal carbonyl compounds and elemental selenium in xylene solvent. Efficient reduction of molecular oxygen to water, in 0.5 M H2SO4 electrolyte, was obtained. This catalyst also showed semiconducting behaviour. XPS-analysis and those results gained by Rutherford backscattering spectrometry (RBS) revealed that oxygen is incorporated in this compound. Transmission electron microscopy (TEM) performed on thin layers deposited on conducting glass or glassy carbon electrodes consisted of amorphous regions and nanocrystal domains with particle sizes ranging from 30 to 40 Angstrom. The probable stoichiometry of the semiconducting catalyst compound resulted to be : (Ru1-xMox)(y)SeOz where 0.02 < x < 0.04; 1 < y < 3 and z approximate to 2y after data analysis obtained with XPS, RBS and TEM.