Platinum catalysts were prepared by impregnation/reduction of two carbon supports with different pore textures: one carbon aerogel and one carbon xerogel. Impregnation with H2PtCl6 was followed by reduction in aqueous phase with NaBH4, filtration, drying and subsequent reduction by H-2. The catalysts were characterized by widely used physico-chemical methods (N-2 adsorption, transmission electron microscopy, X-ray diffraction and CO chemisorption); from these techniques, no significant difference could be detected between the two samples. Actual Pt surface areas measured by coulometry of the electrochemical COads stripping are comparable for both samples. However, the peak position and charge below each electrooxidation peak points towards different fraction of small/large particles within these two samples. In addition, COads stripping shows that a fraction of the Pt particle surface is not electrochemically active. Pronounced differences observed in the specific activity towards O-2 reduction reaction were then explained by structural differences in Pt particles, undetectable by physico-chemical characterization techniques.