This paper treats a problem of nonstoichiometry in TiO2−y thin films deposited by reactive sputtering at controlled sputtering rates. Ion beam techniques, Rutherford backscattering (RBS), and nuclear reaction analysis (NRA) along with X-ray photoelectron spectroscopy have been applied to determine a deviation from stoichiometry y in the bulk and at the surface of TiO2−y layers. The critical review of these experimental methods is given. Defect structure responsible for the electrical resistivity of rutile TiO2 is discussed.