Thin film polymorphs of molybdenum trioxide have been synthesized by RF sputtering. Films deposited on thermally floating substrates are polycrystalline and exhibit preferred orientation. Depending upon the oxygen partial pressure maintained during sputtering, the films can be made to crystallize in either the thermodynamically stable orthorhombic a MoO3 form (unique 2D-layered structure) or the metastable monoclinic /3 MoO3 phase (3D ReO3-related structure). Metastable β films can be converted thermally to the a phase and the transformation appears topotactic. Films deposited on cooled substrates are amorphous. A correlation between the particular phase formed and adatom mobility is noted.