Four electrocatalysts, including a commercial Pt on carbon black (Pt-CB), were compared for performance in the reduction of oxygen. Three of the catalysts were prepared on the basis of the deposition of Pt onto carbon nanotubes using (i) water-in-supercritical CO2 microemulsion (Pt-CNT SCME), (ii) direct supercritical CO2 fluid deposition (Pt-CNT SC), and (iii) water-in-hexane microemulsion (Pt-CNT ME). Cyclic voltammetric studies yielded an electrochemically active surface area for Pt-CNT SCME at 31.1 m(2)/g, which was the largest among all electrocatalysts tested in this work. Hydrodynamic polarization curves for oxygen reduction exhibited that the cell potential of the Pt-CNT SCME catalyst was over 350 mV more positive than the commercial Pt-CB system at 10 A/g of Pt. In chronoamperometric analyses, Pt-CNT SCME catalyst (6.6 x 10(3) A/g of Pt) generated 2.5 times more specific activity at 30 s than Pt-CNT ME (2.6 x 10(3) A/g of Pt) and 5 times more than the commercial Pt-CB (1.3 x 10(3) A/g of Pt). Tafel analysis indicated the exchange current density of 7.87 mu A/cm(2) for Pt-CNT SCME that was significantly higher than the commercial Pt-CB (1.37 mu A/cm(2)).