Two new defect centers have been observed by electron spin resonance (ESR) in rf-sputtered SiO2 films with low metallic impurity content. At 77°K, as-sputtered films have three ESR centers, two HX centers, and an HY center. The HY center, a hole center, has g = 2.008-2.009. The HX center has two components, the HXa center with axial symmetry and with g a∥ = 2.022 and ga⊥ = 2.037; the HXb center with gb = 1.984 which is split by hyperfine interaction with hydrogen. The hydrogen hyperfine interaction has axial symmetry, Ab∥ = 30.2 G and Ab⊥ = 63.6 G. Both the HX and HY centers show hyperfine splitting due to interaction with Si29. Annealing of as-sputtered films in hydrogen-containing atmospheres at 350°C eliminates the HY center, leaving the HX center. Annealing in N2 or O2 at 550°C also eliminates the HY center; the resulting HX resonance is identical to that obtained after hydrogen anneal. The activation energy for the appearance of the HX center is 0.036 eV when samples are cooled to 77°K. From similarities in hyperfine interactions, activation energies, and g values, it is suggested that the HY center is the silicon analog of the Al center in SiO2. It occurs in oxygen-deficient SiO2 and is due to removal of a nonbonding electron from an oxygen which bonds Si+3 to Si+4. The HY center is converted to the HX center by bonding with hydrogen which can be provided by H2 from the annealing ambient or by the H atom in Si-OH incorporated in SiO2 films. © 1974 American Institute of Physics.