MoO3-Bi2O3 thin films are grown by CVD on SiO2/Si(100) and Al2O3 using BiPh3 and MoO2(dpm)(2) as precursors. The composition, microstructure and morphology of the samples can be suitably controlled by the proper combination of synthesis and thermal treatment conditions. The chemical characterization of the obtained films is carried out by XPS and FTIR, while the microstructure and surface morphology are investigated respectively by XRD and AFM. Preliminary measurements on Bi-doped MoO3 films indicate that they could be used for the detection of scarcely reducing gases. (C) Published by 1998 Elsevier Science S.A. All rights reserved.