Photoactive thin films of tungsten disulfide have been prepared by sulfurization of WO3 layers, Tungsten trioxide films were deposited on heated quartz and glassy carbon substrates by gaseous reaction of W(CO)(6) with oxygen. Subsequently, the oxide films were treated at 700 degrees C in a gaseous sulfur atmosphere either in a flowing system or in evacuated and sealed quartz ampoules. The films were investigated by X-ray diffractometry, scanning electron and scanning tunneling microscopy. The photoactivity was measured using time-resolved microwave conductivity. Films consisting of the 2H-WS2 platelets, highly textured and oriented with their c axis perpendicular to the substrate, were found, In addition, the phase relations in the phase triangle W-O-S were studied by thermochemical equilibrium calculations using the Gibbs free energy minimization technique. The inferred predominance area diagram log p(S2), vs. log p(SO2) confirms the high stability of WO29 and WS2 as observed under related experimental conditions. The effect of the nickel on the orientation of the 2H-WS2 crystallites obtained after sulfurization of the WO3 films can be explained by a flux on the basis of the binary Ni-S phase diagram or surfactant mediated growth effect.