Supercritical deposition was used to synthesize Pt/carbon aerogel (Pt/CA) electrocatalysts. CAs with average pore diameters of 6, 8 and 19 nm (CA6, CA8 and CA19, respectively) were synthesized and impregnated with Pt(cod)me2 precursor using supercritical carbon dioxide followed by the thermal conversion of the Pt precursor into its metallic form under N2 atmosphere at various temperatures between 200 and 1000 °C. All of the prepared CAs have high surface areas with very sharp pore size distributions. XRD and TEM results show increased Pt particle size with increasing conversion temperature with a homogenous distribution of nanoparticles on the CA supports. Cyclic voltammetry was used to determine the effect of CA pore properties on electrocatalytic activity. At a conversion temperature of 400 °C, the highest and lowest electrochemical surface area values were obtained for Pt/CA19 and Pt/CA6 of (126 and 36 m2/g, respectively). Furthermore, Pt/CA19 showed good mass activity whereas Pt/CA6 and Pt/CA8 had lower activity values towards the Oxygen Reduction Reaction (ORR). The mass activity values for Pt/CA19 increased with increasing conversion temperature, except for the sample converted at 1000 °C which exhibited the lowest mass activity. The specific activity increased significantly with the conversion temperature up to 600 °C which gave a value six times that obtained at 200 °C. At 800 °C, the specific activity decreased slightly, probably due to a change in the CA structure at this elevated conversion temperature. The Pt/CA19 sample converted at 600 °C exhibited the best performance with a mass activity of 0.1 A/mgPt and a specific activity of 0.24 mA/cm2. Display omitted •Carbon aerogels with different average pore sizes were loaded with Pt nanoparticles via supercritical deposition method.•The average Pt particle size could be controlled by changing the conversion temperature.•Pt nanoparticles were homogeneously distributed on the surface of carbon aerogels with a narrow particle size distribution.•High ESA and electrocatalytic activity were obtained for carbon aerogel with the larger average pore size.•Highest electrocatalytic activity was obtained for Pt nanoparticle size of 1.9 ± 0.2 nm.