In a CO-saturated water/2-propanol mixed solvent, K2PtCl4 salt can be radiolytically reduced either into molecular carbonyl clusters of the [Pt-3(CO)(6)](2-)(n) series or into colloidal metal particles by varying the CO:Pt ratio, The reduction occurs by a combined effect of CO and radicals produced by radiolysis, giving rise to unusually high reduction yields, The colloidal platinum and also the molecular clusters can easily be impregnated on carbon black, The particle size of the supported catalyst prepared in this way lies in the 2-3 nm range and is independent of the catalyst loading and support surface area, even for high loadings (3-nm particles have been deposited up to a 60 wt% loading on a Printer powder), These catalysts are found to be efficient for methanol or hydrogen electrooxidation and for oxygen electroreduction. The CO-stabilized colloids have been characterized by electron microscopy and UV-visible spectroscopy,