Viscosity coefficients were measured, by Lillie's method, of titania opacified glasses of more than ten compositions, some containing B2O3 and some not containing it, at several temperatures between 560°and 700°C., with the object of explaining the strong opacifying power, that is the facility of depositing small titania particles by heat treatment, of the glasses containing B2O3. Comparing inner bond energies and softening points, calculated from these measurements, it is proved that this strong opacifying power is attributed to the lowering of inner bond energy, and addition of B2O3 must be of fairly large quantity.