Molybdenum oxide samples were prepared using different temperatures and atmospheric conditions by metallo-organic decomposition processes and were characterized by XRD, SEM and DRS UV/Vis and Raman spectroscopies. Variation in the synthesis conditions resulted in solids with different morphologies and oxygen vacancy concentrations. Intense characteristic Raman bands of crystalline orthorhombic alpha-MoO3, occurring at 992 cm(-1) and 820 cm(-1), are observed and their shifts can be related to the differences in the structure of the solids obtained. The sample obtained under nitrogen flow at 1073 K is a phase mixture of orthorhombic alpha-MoO3 and monoclinic beta-MoO3. The characterization results suggest that the molybdenum oxide samples are non-stoichiometric and are described as MoOx with x < 2.94. Variations in the reaction conditions make it possible to tune the number of oxygen defects and the band gap of the final material. (C) 2010 Elsevier Ltd. All rights reserved.