A detailed electrochemical characterisation of platinum-palladium nanoparticles prepared by reduction with hydrazine of H2PtCl6 and K2PdCl4 in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ(R)30)/n-heptane is reported. XPS, AAS and UV-vis experiments were carried out to determine the atomic composition of the bimetallic nanoparticles obtained in the whole composition range. Cyclic voltammograms of clean nanoparticle alloys were obtained after a controlled decontamination procedure of their surfaces. CO adsorption-oxidation and dissociative adsorption of formic acid and methanol were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. FTIRS experiments of adsorbed CO were also carried out giving information on the relative amount of linearly and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. Experimental results show how the electrochemical adsorption behaviour and electrocatalytic properties of the particles are under the control of their composition and, in a case studied in this work, of their size. (C) 2003 Elsevier Science B.V. All rights reserved.