Optical, structural and electronic properties of amorphous and crystalline molybdenum oxide thin films have been investigated. As-deposited amorphous films got crystallized into a layered orthorhombic phase on annealing at 350degreesC. Refractive index (n) and extinction coefficient (k) of as-deposited films and films annealed at 150degreesC, 240degreesC and 350degreesC have been calculated using reflectance and transmittance data. Spectral dependence of absorption coefficient has been explained on the basis of charge transfer transition mechanism. Optical band gap of amorphous MoO3-x is 3.16 eV and it has increased by 0.11 eV on crystallization. XPS core level analysis reveals the presence of Mo+4, Mo+5 and Mo+5 oxidation states in amorphous films, proving oxygen deficiency in as-deposited films. Same studies on crystalline films show the presence of only Mo+6 states. Valence band spectrum of amorphous films reveal emission from Mo4d levels, which is absent in crystalline films. Complete correlation is seen between the optical properties and XPS data. (C) 2004 Elsevier B.V. All rights reserved.