Thick tungsten nitride films have been deposited on GaAs substrates using a reactive rf-sputtering system. The optical indexes of the WNx films (determined by in situ kinetic ellipsometry) and their nitrogen contents (determined by Rutherford backscattering), have been determined versus the pressure ratio PN2/Ptot. Using reactive sputtering conditions which provide highly nitrogenated films, W/WNx twenty period multilayers with nanometric layer thicknesses have been deposited. The experiment has also been monitored in situ by kinetic ellipsometry at 1.96 eV and the multilayer has been analyzed ex situ by grazing x-ray reflection measurements at 1.54 Å. The composition of the different layers has been determined precisely by Rutherford backscattering analysis. Both W–WNx and WNx–W interfaces in the multilayer appear very sharp by kinetic ellipsometry. This is confirmed by the occurrence of well defined Bragg peaks on the grazing x-ray reflectivity curves in spite of the low density contrast between W and WNx layers.