Carbon supported catalysts for oxygen reduction in acidic media based on ruthenium and selenium have been prepared by thermolysis of Ru-3(CO)(12) in organic solvents. The mass specific activity of these catalysts is higher than that of the unsupported samples. The optimum loading of the supporting carbon with catalyst has been found to be about 10%; higher loadings lead to only a slight increase in activity. As is evident from TEM, agglomeration of catalyst particles occurs, however, these agglomerates are homogeneously distributed over the supporting carbon. The activity is compared with commercial high surface area Pt/carbon and Ru/carbon catalysts. Characterisation of the catalyst with XPS and XRD indicates that it consists of a ruthenium core, which is surrounded by an amorphous shell containing Ru in various oxidation states, selenium and oxygen as well as high amounts of carbon. The real structure of the shell is unknown. After heat treatment, the activity of the catalyst towards oxygen reduction is slightly enhanced.