Pt-Au nanoparticles supported on high area carbon were prepared by simultaneous reduction of Au and Pt precursors and by reduction of Pt precursor on already prepared Au nanoparticles. The first method produced a solid solution of Pt in Au containing similar to 5% Pt with the remaining Pt on the nanoparticles' surface. For the Pt:Au precursor ratio of 1:4 and 1:9, the surface ratio was found to be 0.70:0.30 and 0.55:0.45, respectively. By the second method with the Pt:Au precursors ratio of 1:12, the surface ratio was 0.30:0.70. The voltammetric peaks of Pt-oxide reduction and CO(ads) oxidation demonstrated electronic modification of Pt by Au in all catalysts. With decreasing Pt:Au surface ratio the activity for HCOOH oxidation increases and surface coverage by CO(ads) decreases. The highest activity under potentiodynamic and quasi steady-state conditions without poisoning by CO(ads) was observed for the catalyst with the lowest Pt:Au surface ratio. Chronoamperometic test showed that its high catalytic activity is associated with a high deactivation rate. It was postulated that too strong adsorption of a reactive or non-reactive intermediate caused by electron modification of Pt by underlying Au, is responsible for the deactivation. This result stresses that high Pt dispersion, necessary for promotion of the dehydrogenation path in HCOOH oxidation, can produce too strong adsorption of intermediates causing deactivation of the catalyst. (C) 2011 Elsevier B.V. All rights reserved.