Polycrystalline MoO3 thin films have been prepared by oxidation at high temperature of molybdenum compound layers deposited by chemical vapour deposition (CVD) from molybdenum hexacarbonyl Mo(CO),. In this paper is presented the experimental procedure to prepare polycrystalline MoO3 coatings by annealing at various temperatures and in various annealing environments of "black molybdenum" or "reflective molybdenum" thin films. From X-ray diffraction measurements the structure of the films has been determined. The optical constants II and k of films were calculated by numerical iteration, using the Fresnel laws and dispersion relations of n and k. From these results the optical gap E-g has been deduced. AU the samples exhibited the electrochromic effect.