The structural properties of annealed (ZnO)(x), (CdO)(1-x) thin films are studied by x-ray diffraction methods. The films were obtained by spray pyrolysis using different values for the nominal composition (x), and then they were annealed at 450 degreesC from 0 to 120 min. The structural analysis confirms previous results on the formation of a homogeneously mixed oxide semiconductor, but in which crystalline and amorphous phases co-exist for both CdO and ZnO. In this work, we show that for annealed films there is a strong interaction between the amorphous, the hexagonal ZnO and the cubic CdO phases regarding the lattice constants and the crystallite growth rate. In the annealed films, for x < 0.5 the optical behavior is mainly controlled by the CdO phase, so that there is a reduction of the effective band-gap of the material when the CdO crystallite size is increased, possibly due to grain size effects. On the other hand, for x > 0.5 the band-gap behavior is mainly determined by the variation of the ZnO crystallite lattice parameters and the relative volumetric concentration of amorphous and crystalline ZnO in the film. (C) 2002 Elsevier Science B.V. All rights reserved.