Electrical and optical properties of reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in sputtering atmosphere. The films with 5200–6600 A˚ thick were prepared under a constant pressure of 5×10−3Torr using a mixture of Ar and 4–50% O2. Electrical resistivity and spectral transmittance of the films formed depend extremely on the oxygen concentration in the atmosphere. The films formed in an atmosphere containing less than 13% O2are semiconductive, have a resistivity of 10−3–103&OHgr; cm, and are blue colored. The films formed in an atmosphere containing more than 16% O2are transparent and have a resistivity of 107–1011&OHgr; cm. Optical band gap of the films formed under a sputtering atmosphere containing 8–50% O2is almost constant and ranges 3.00–3.03 eV. The carrier concentration and Hall mobility of semiconductive films and the refractive index of insulating films were also measured. The presence of a critical oxygen concentration for the formation of either semiconductive or insulating films has been suggested.