Abstract The crystallization kinetics of silica glass (Corning 7940) and titanium oxide-silica glass (Corning 7971) was investigated. Special attention was paid to prevent the contamination of the surface of the samples during their heat treatment. It was found that the thickness of the crystallized layer depends linearly on the time of heat treatment for both types of the investigated glasses. The beginning of the crystallized-layer growth is preceded by the quite considerable induction period. The temperature dependences of the induction period and of the rate of crystal growth for silica glass coincide practically and are characterized by the activation energies E τ ≈ E u ≈ 160 kcal/mol. For titanium oxide-silica glass E τ > E u . The measured value of the activation energy of viscous flow differs considerably from the activation energy of the crystal growth rate. This may be evidence of the fact that the crystallization and shear viscosity depend on different kinetic units of silica glass.