Thin films of Mo and W mixed oxides were achieved by reactive sputtering, assisted by the selective sublimation processing technique. Deposition of the film resulted in nucleation of Mo oxide in a Mo-W-O amorphous matrix, then the film underwent partial crystallization and oxidation on annealing. A two-phased nanostructure was achieved, featuring substoichiometric MoO3 flakes dispersed over a porous W-Mo-O film. Diffusion of Mo ions toward the film surface and sublimation of MoO3 were observed, the latter resulting in porosity increase of the layers through annealing. This behavior is interesting because it allows one to establish the content of residual Mo in the W-Mo-O film. Moreover, the presence of dispersed Mo and W anions in the WO3 or MoO3 phases inhibited grain coalescence upon annealing. The films were tested as chemo-resistive sensors towards CO at concentration level useful for environmental monitoring. (C) 2003 American Vacuum Society.