The electrochemical properties of platinum hydrosol. obtained by the reduction of H2PtCl6 by sodium citrate (NaH(2)Cyt), were studied for the first time. Hydrosol particles of mean diameter d(c) approximate to 2.0 nm, as estimated from UV-vis absorbance spectra. were immobilised on a Au support under anodic polarisation to form stable Pt-col/Au electrodes. High Pt coverages were found from a combination of cyclic voltammetry and scanning tunnelling microscopy data. Immobilised Pt particles (most probably. small uniform aggregates) with tl, of about 4.5 nm and a spread of size distribution ca. 1.5 nm demonstrated pronounced adsorption of hydrogen and oxygen similar to pure Pt materials, high electrocatalytic activity with respect to formic acid oxidation as well as a relatively high tolerance towards CO poisoning.