Changes in the refractive index and molar volume of thermally grown SiO2 films with the growth or annealing (in inert ambients) temperature are characterized by 0.01 eV activation energy. It is suggested here that with increasing temperature the rotational angles between SiO4/2 tehahedra decrease and/or the bridging bond angles increase resulting in increased intertetrahedral distances and molar volume (decreased refractive index). The activation energy of the generation of P-b centers during annealing in vacuum is 0.25 eV, indicating that strained Si-O bonds with small bridging bond angles (<120 degrees) and enlarged bond lengths (>0.160 nm) are involved. The density of these bonds is reduced by He in the oxide so that annealing in He results in reduced density of P-b centers.