Amorphous non-stoichiometric molybdenum oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of molybdenum dioxo tropolonate complexes on Si(100) substrates. Photodeposited films were characterized by X-ray photoelectron spectroscopy (XPS) and the surface morphology examined by Atomic Force Microscopy (AFM). It was found that as-photodeposited films are uniform with rms surface roughness of 231 nm and contain non-stoichiometric oxides (MoO3 - x). The results of XRD analysis showed that post-annealing of the films in air at 350 C transforms the sub-oxides to α-MoO3 phase with a much smoother surface morphology (rms = 164 nm). Both, the as-photodeposited and annealed MoO 3 - x films exhibit good optical quality with transparency in the visible region better than 80%. Gas sensing tests reveal that annealed MoO 3 - x films exhibit a better response than as-deposited films towards 50 ppm ammonia at an operating temperature of 350°C.