Ti-Cu binary films co-sputtered in vacuum are catalytically active for the oxygen reduction in 0.5 M H(2)SO(4). The activity for the oxygen reduction reaction (ORR) increased with increasing the Cu-content in the Ti-Cu films and it reached to a maximum with the copper composition up to 90 aft The constant Tafel slope of similar to 190 mV/decade which is comparable to that obtained on pure Cu films indicates that the active sites for oxygen reduction is copper sites. Through investigation of Tafel polarization, the Ti-Cu films revealed a constant Tafel slope (i.e., 190 mV/decade) similar to that of ORR on the pure Cu film. This infers that the electrochemical reduction of oxygen is predominated on the Cu-sites in the film. In the cyclic voltammograms, the strong broad peak should have arisen from the oxidation of Cu to Cu(+) and Cu(2+) ions. This oxidation indicated that the Ti-Cu films are unstable and the Cu-component is susceptible to dissolution in 0.5 M H(2)SO(4). This dissolution caused a loss of catalytic activity in the films. Preparing the Ti-Cu films enriched in Ti will stabilize these films to prevent the Cu-dissolution. (C) 2009 Elsevier B.V. All rights reserved.