In this work, we report on the structural investigation by transmission electron microscopy of molybdenum-based mixed oxides for use as gas sensitive films. In particular, sol-gel thin films of molybdenum-titanium and molybdenum-tungsten oxides, deposited by spin coating on silicon substrates, have been analyzed by selected area electron diffraction, conventional, and high-resolution transmission electron microscopy. Our results show that the relative percentage of the molybdenum precursors used during the sol-gel deposition controls the morphology of the films, by modifying the size and the arrangements of the polycrystalline grains on the substrate.