Electron microprobe and helium ion backscattering are used to measure the stoichiometry of rf‐sputtered SiO2films at a precision of ?1%. Both oxygen‐excess and oxygen‐deficient films occur. Optical absorption and electron spin resonance characterize atomic defects in the films. The correlation between stoichiometry and atomic defects is poor. The reproducibility of composition and of atomic defects from run to run, when sputtering conditions are held constant, is good.