The kinetics of O2 electroreduction on Pt in concentrated trifluoromethane sulfonic acid (TFMSA) have been investigated at elevated temperatures up to ∼90°C, using microelectrode technique, and the results compared with those previously obtained by similar measurements in concentrated H3PO4 . The activation energies for the reaction in ∼9.5M, 6.0M, and 3.0M TFMSA were, respectively, 17 kJ mol−1, 20 kJ mol−1, and 29 kJ mol−1, as compared to the value of ∼66 kJ mol−1 in 98 weight percent (w/o) H3PO4 . The exchange current densities and the diffusion‐limiting current densities in concentrated TFMSA solutions were, respectively, three and two orders of magnitude higher than the corresponding quantities obtained in 98 w/o H3PO4 . The solubilities and diffusion coefficients of O2 in TFMSA solutions have been evaluated, and their role in the current‐potential behavior for O2 reduction in these electrolytes is discussed.