Films of nominal composition NixCo3-xO4 (x = 0.3 1.3, 1.8) prepared by spray pyrolysis have been characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the film with x = 0.3 is monophasic, presents a spinel-related structure and contains Ni2+, Ni3+, Co2+ and CO3+. In the films with x = 1.3 and 1.8 the preparation leads to the formation of significant amounts of NiO and Coo which are segregated to the surface of the sprayed films. The effect on the reducibility of the different materials under Ar+ sputtering has also been investigated. The film with x = 0.3 was found to be the most difficult to reduce, whilst the films with x = 1.3 and 1.8 were reduced to metallic Ni, metallic Co, and CoO. (C) 2003 Elsevier B.V. All rights reserved.