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Concentrated aqueous perfluoroethylene-1,2-bis-phosphonic acid, (OH)2OPCF2CF2PO(OH)2, has been investigated as an alternative to phosphoric acid as the electrolyte in the H2/02 acid fuel cell. Phosphoric and bis-phosphonic acid solutions were equilibrated with pure 02 (P = 1 atm) and were used in (i) chronoamperometry experiments performed with a Pt microelectrode to estimate the °2 diffusion coefficient, D, and solubility, C, and (ii) 02 electroreduction polarization studies done with a gas-fed Teflon-bonded carbon electrode with a high surface area Pt catalyst in a microfuel cell. At 22°C in 85% bis-phosphonic acid, C was found to be 3.4 0.3 X iO mol/liter and D was 5.0 0.5 X i0 cm2/s as compared with C = 2.8 0.3 X 10 mol/liter and D = 1.2 0.1 X 10-6 cm2/s in 85% phosphoric acid at T = 22°C. The overpotential for 02 reduction on the microfuel cell cathode was smaller in 85% bis-phosphonic acid than in 85% phosphoric acid for current densities up to 200 mA/cm2 at 100°C. This indicates that the oxygen electroreduction kinetics on Pt increased when phosphoric acid was replaced with the perfluorinated bis-phosphonic acid. At higher current densities ( 200 mA/cm2) where mass transport may influence cathode performance, the performance with bis-phosphonic acid was not as good as with phosphoric acid at 100°C, which is consistent with the higher viscosity and lower D found for bis-phosphonic acid relative to phosphoric acid at 22°C. As the temperature was raised from 100 to 200°C, the cathode performance improved with the bis-phosphonic acid electrolyte. The enhanced 02 reduction kinetics with the bis-phosphonic acid persisted in the fuel cell environment at temperatures up to 200°C for 500 h which demonstrated a major improvement in stability for a high performance perfluorinated fuel cell electrolyte used at higher temperatures such as 200°C.