The reduction of oxygen on electroless deposited platinum on a freshly polished titanium electrode was studied in aqueous 0.1 M HClO4 and 0.2 M NaOH solutions, utilising the rotating-disc electrode technique. TEM studies show that at short deposition times Pt is highly dispersed on titanium. At longer deposition times the degree of dispersion decreases due to coalescence and agglomeration processes and larger clusters are formed. The kinetics of oxygen reduction were affected substantially by the time of Pt deposition on Ti at the open-circuit potential. A kinetic analysis of the current -potential curves was made in the kinetically controlled region in the form of Tafel plots and in the mixed kinetic-diffusion control region in the form of Koutecky-Levich plots. The catalytic activity of the Pt crystals decreases with increasing deposition time. The results were interpreted in terms of the effect of the Pt crystal size. The larger the Pt particles the lower the catalytic activity. The role of titanium oxide formation and the electronic effects were also discussed.