Molybdenum oxide thin films were prepared by pulsed laser deposition. The as-deposited films were dark. Annealing the films in air at 400°C resulted in transparent films. These films were further annealed in vacuum at 300 and 400°C to investigate thermocoloration. The structural, chemical, and optical properties of the films were determined. All films were predominantly amorphous. The air-annealed films were nearly stoichiometric. However, after annealing in vacuum at 400°C, the films became oxygen-deficient. The transmittance of the films progressively decreased as the vacuum-annealing temperature increased, indicating stronger coloration of the films. The optical constants were determined from the transmittance measurements. Both the refractive index and extinction coefficient increased upon vacuum annealing. However, the band gap slightly decreased after vacuum annealing.