The study of the deposition process of chromium oxide thin films (Cr2O3) by magnetron sputtering is the main purpose of this work. A pure chromium target biased by a d.c. voltage is sputtered in a reactive atmosphere of argon and oxygen. A systematic study of the deposition parameters like oxygen partial pressure has made it possible to follow with precision the mechanisms of target poisoning. On each part of the hysteresis loop, the amount of reacting oxygen and the number of sputtered chromium atoms have been calculated. It has been compared with the results of X-ray photoelectron spectroscopy analysis. The evolution of the crystallization of the coatings has been studied depending on substrate temperature by X-ray diffraction analysis. An optimal operating point for deposition of stoichiometric oxide coatings has been determined. In these conditions, for temperatures above 300 degrees C, the films consist of pure well-crystallized Cr2O3 and present the characteristic green colour of bulk Cr2O3.