The thermal expansion coefficient (TEC) of binary glasses SiO 2 -TiO 2 decreases as the TiO 2 concentration grows. At concentration of TiO 2 in the range of 4–6 mol% (in dependence of the applied synthesis method) TEC becomes negative; at concentration of 8–11 mol% devitrification occurs. A combination of photoluminescence (PL) and IR reflection techniques were applied for studying the evolution of point and structural defects in the range of TiO 2 concentration from zero to phase separation. The precision measurements of density showed its drop at TiO 2 content of 7.9 mol% thus evidencing the conversion of glass into a polycrystalline substance consisting of SiO 2 and TiO 2 crystallites. In the TiO 2 concentration range of compatibility of oxides, an ensemble of point defects comprises ≡Si–O – and ≡Ti–O – centers as well as ≡Si–Si≡ and ≡Ti–Ti≡ neutral oxygen-vacancies. At the dopant concentration specific for phase separation (7.9 mol%), the PL-active groups of Si–Ti go away, while the IR-detectable Si–O–Ti bridges remain stable.