The reduction of partially- and fully-oxidized Cu films was carried out by vacuum annealing, and the reduction mechanisms were investigated in situ by XPS. For a partially-oxidized Cu film, CuO was reduced to CU2O around 380 K, and the Cu2O concentration decreased with increasing annealing temperature and fell below the XPS detection limit at 673 K. For a fully-oxidized Cu film, CuO started to form Cu2O around 473 K, and Cu began to appear at 673 K. The results for a partially-oxidized film are interpreted in terms of oxygen diffusion into bulk Cu rather than oxygen desorption into vacuum. (C) 2002 Elsevier Science B.V. All rights reserved.