The primary objective of this program is to define and develop electrocatalysts and supports for the positive electrode of moderate-temperature, single-unit, regenerative fuel cells. Both alkaline and solid polymer proton-exchange membrane (PEM) electrolytes have been investigated, with emphasis on the oxygen electrode in both systems. In alkaline electrolyte, the testing has been performed primarily on a half-cell basis with a floating-electrode cell. With solid polymer electrolytes, testing has been performed primarily with complete PEM fuel cells using duPont Nafion 117. Results are presented primarily for Pt, Ir, Rh and NaxPt3O4 catalysts. In 30% KOH at 80°C the bifunctional performance (iR free) at 200 mA/cm2 observed for Pt/RhO2 is 0.875 V in O2 reduction and 1.438 V in O2 evolution (vs. RHE); and for NaxPt3O4, 0.868 V in O2 reduction and 1.444 V in O2 evolution (vs. RHE). For the PEM cell at 80°C, the bifunctional performance (iR included) at 500 mA/cm2 observed for Pt/Pt-IrO2 is 0.723 V in fuel-cell mode and 1.587 V in electrolysis; and for Pt/Pt-NaxPt3O4, 0.740 V in fuel-cell mode and 1.697 V in electrolysis.