Thin films of crystalline alpha -MoO3 with thicknesses between 35 and 150 nm were prepared on Si (100) wafers by spin-coating of peroxo-polymolybdic solutions. Variations of the morphology of the films with spin-coating conditions and thermal treatments applied afterwards were investigated by means of confocal laser Raman spectroscopy, X-ray diffraction and scanning electron microscopy. Thickness and uniformity of the films, and size, anisotropy and orientation of alpha -MoO3 crystals with respect to the Si substrate can be tuned by adequately adjusting the concentration of the precursor solution, the solvent, and the spin-rate during spin-coating. Size, anisotropy and orientation of crystals are nevertheless interconnected and cannot be tuned independently. The three variables are themselves dictated by the thickness of the films. However, the choice of the solvent used for the precursor solution seems to provide some additional flexibility in the tuning of the morphology of alpha -MoO3 crystals, while the minimum thickness required to obtain stable crystalline alpha -MoO3 films under the conditions we used for calcination is estimated to be 35 nm.