This work reports an electrode material containing molybdenum phosphate that shows a very interesting behavior as an electrocatalyst for the reduction of oxygen to water (ORR) in acid medium. The material is electrodeposited on a glassy carbon electrode from a solution of Mo(VI) and Co(II) in phosphate buffer at pH = 7. Cobalt is required to obtain a compact and uniform film. It is a non-crystalline solid conformed by metal oxide and phosphate groups that, according to XPS analysis, contains mostly Mo(VI) and a small fraction of Mo(V). Even though most of the Co-containing phase dissolves in acid, the film keeps its integrity and shows stable ORR activity in acid at very low overpotentials (from -0.3 V). The observed current density reaches a very small limiting value (similar to 2 mu A cm(-2)) that is independent of the mass-transport conditions and could be associated to the small fraction of Mo(V) centers that function as ORR active sites. Although the film is thermally stable up to temperatures of 400 degrees C, by heating the material at temperatures above 100 degrees C it looses its unusual ORR activity due to the complete oxidation of Mo(V) to Mo(VI). (C) 2011 Elsevier B.V. All rights reserved.