The catalytic activity of Pt nanoparticles supported by zeolite-type mesoporous Cs2.5H0.3W12O40 solid super-acid towards oxygen reduction has been explored. Pt(IV) impregnated Cs2.5H0.3W12O40 was prepared by titration of an aqueous solution of phosphotungstic acid and a known quantity of H2PtCl6 with a solution of cesium carbonate. The H2PtCl6 impregnated insoluble salt was subsequently chemically or electrochemically reduced to form supported Pt nanoparticles. HRTEM micrographs show that the reduced composites Pt-Cs2.5H0.5PW12O40 (Pt-Cs2.5PW12) contains Pt nanoparticles of average dimensions 2-5 nm, embedded into the porous structure of the insoluble salt. RDE and RRDE studies and fuel cell polarization data performed using thin layer composite films of Pt-Cs2.5PW12 chemically or electrochemically reduced show that the catalyst is very stable and rather efficient for oxygen reduction (ORR). The oxygen reduction wave is shifted by 60-70 mV towards more positive potentials with respect to the one obtained using electrodes prepared with a standard catalyst such as E-Tek 20% Pt-Vulcan. Analysis of the voltammetric curves demonstrates that the electrocatalytic activity of the composite electrodes is higher than that of the standard catalyst. This indicates that the solid state super-acid Cs2.5H0.5PW12O40 acts as co-catalyst by providing a proton rich environment in the vicinity of the Pt nanoparticles that enhances their catalytic activity towards oxygen reduction. (C) 2011 Elsevier B.V. All rights reserved.