The objective of the study was to investigate the platinum particle size effect on the catalytic activity for oxygen reduction reaction in alkaline solution. Various calibrated Pt particles on Vulcan were used. The catalytic powders were characterised by TEM (transmission electron microscopy) and H-electrochemical adsorption. For oxygen reduction, a small loss of catalytic activity with the particle size decrease is demonstrated: the specific activity is three times lower for very small particles. We have also made a study by RRDE which completes and fully confirms the catalytic activity results. This study has shown that the two-electron path is important on carbon, almost negligible on Pt: a comparison between a pure carbon powder and this powder with deposited Pt particles has shown that the total current density is very small on pure carbon at our working potentials. Although the RRDE study does not permit a fully complete analysis of the minor pathways, that is to say how much HO2- produced on carbon is reduced on Pt and how much HO2- is directly produced on Pt particles, it appears that the ORR is predominantly governed by the activity of Pt particles dispersed on the carbon support.