To find the best operating conditions and oxygen electrode catalyst of the PEM unitized regenerative fuel cell (URFC) system, we examined the effect of reaction conditions and the performance of several bifunctional electrocatalysts for oxygen electrode in a single cell PEM URFC system. When Pt black was employed for both electrodes, the URFC performance improved with increasing cell temperature up to 70 degreesC and catalyst loading up to 6.7 mg cm(-2). Among the catalysts prepared in the present study for oxygen electrode, fuel cell performance decreased in the order of Pt black > PtIr > PtRuOx > PtRu similar to PtIr > PtIrOx, whereas, water electrolysis performance decreased in the order of PtIr similar to PtIrOx > PtRu > PtRuIr > PtRuOx similar to Pt black, showing significant performance improvement by addition of second metals such as Ir or IrOx to Pt black. Consequently, PtIr had the best URFC performance with high round-trip efficiency and stability during the cycling operation of the URFC system. Therefore, PtIr is promising as an oxygen electrode of the PEM URFC system. (C) 2004 Elsevier Ltd. All rights reserved.