Different concentrations of WCl6 have been added to three kinds of coking coals before their pyrolysis. The rank of the coal and mainly the volatile matters removed during the heat treatment are of great influence on the transformations of tungsten hexachloride. It first reacts with oxygen present in the starting coal to transform into WO2.9 between ambient and 550 degrees C. From 550 degrees C to 750 degrees the trioxide is reduced to WO2 and a fraction reacts with Ca to form CaWO4 as shown by X-ray diffraction. The total amount of W introduced in the initial mixture is recovered in the resulting green-coke and the total amount of chlorine is removed as HCl. After steam activation, WO2 is oxidised into W24O68.