Electron spin resonance studies of SiO2 films prepared by rf sputtering have revealed the presence of defects intrinsic to the silica network, such as the E′ center and peroxy radical. Films sputtered in a mixed ArN2 atmosphere were found to contain bridging nitrogen centers; the spectroscopic parameters are consistent with SiNSi structures. A low concentration of NO2 molecules has been identified in a second series of films deposited in Ar with resputtering. Although the molecules are immobilized in the silica network, they undergo thermally-activated librational motion. © 1985.