The gas sensing proper-ties of substoichiometric WO3 films have been investigated when activated with some gold monolayers sputtered on their surface and in the as-deposited form. These films were deposited on silicon substrates, in two steps : (i) chemical vapor deposition of tungsten films using W(CO)(6) vapors in a horizontal cold wall reactor at a pressure of 13.3 Pa and temperatures between 773 and 823 K, and (ii) oxidation of the tungsten films in air at 873 K for 10 min. For the tests, the substoichiometric WO3 films were packed in classic dual in-line packages. The resistance variations of these configurations caused by changes in their environment were monitored. Reversible changes, of the order of several kilohms, were observed in th presence, or upon removal H-2, at concentrations down 100 ppm, the magnitude of which depended on the concentration and the temperature measured. The response times measured were also dependent on temperature and H-2 concentration, and were of the order of some minutes.