One of the main challenges toward reversible metal-air batteries and regenerative fuel cells is the bi-functional water/oxygen electrocatalyst component. At present, no single metal oxide can catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with low overpotential between them. Here, the composite concept is employed to address this problem in conjunction with well-defined quantification of the onset of both reactions utilizing thin-film rotating disk electrode technique. This work is built up on the idea of utilizing a good ORR catalyst (e. g. PdO) and a good OER catalyst (e. g. RuO2) in alkaline solution. Four electron mechanism and two electron mechanism are implied for the ORR catalyzed using PdO containing compounds and RuO2, respectively. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.019302jes] All rights reserved.