The thermal oxidation WSi2 films has been studied. Thin films of WSi2 were deposited by sputtering on (100) silicon and on oxidized silicon, and then oxidized in dry O2 and in steam atmospheres at 1000, 1100, and 1200 °C for times up to 30 min. In all cases the dry O2 oxidation resulted in films of poor quality. The films had rough surfaces, with WO3 present in powder form. However, steam oxidation gave SiO2 films of good quality. For WSi2/Si as well as WSi2/SiO2/Si samples, as the oxidation proceeded, WSi2 reacted with steam to form SiO2 and W5Si3. The rate of oxidation and activation energy were different for the two types of substrates. For WSi2 films deposited directly on Si, the oxidation process was significantly modified because Si atoms rapidly diffused through the silicides and took part in the oxidation process, and thus affected the conversion of WSi2 to W5Si3. However, with WSi2 films deposited on oxidized Si, the absence of free Si atoms resulted in the faster transformation of WSi2 into W5Si3.