The objective of the study was to investigate the effect of particle size on the catalytic activity for oxygen reduction reaction at platinum/recast ionomer interface. To obtain experimental evidence of this effect, porous electrodes of well-defined geometry and very well calibrated Pt particles on graphite were used. The catalytic powders were prepared by cationic exchange and characterised by TEM and H and CO electrochemical adsorptions. For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed. This activity loss is correlated to the stronger adsorption of oxygenated species under inert atmosphere and during oxygen reduction. No effect of the inter-particles distance was found even when the particles are 1.2nm in diameter and about 10nm away; the use of graphite powder also prevents a too strong shielding effect of the catalyst support.