An electron spin resonance (ESR) study has been carried out of point defects generated in standard thermal SiO2 on (100) Si during vacuum annealing in the temperature range T-an=950-1250 degreesC, including the predominant exclusive S center in addition to the familiar E-gamma', E-delta', and EX defects. The latter only appear after 10-eV optical excitation. The S and E-gamma' density is found to increase monotonically with T-an, while EX and E-delta' detectivity fades for T(an)greater than or equal to1050 and 1200 degreesC, respectively. Over broad T-an ranges, the generation of all three defects S, E-gamma', and E-delta' appears thermally activated (Arrhenius type) with a common activation energy E(a)approximate to1.6 eV. Large defect densities may be reached, i.e., [S] up to similar to1x10(15) cm(-2) for T-an=1250 degreesC, typically one order of magnitude larger than [E-gamma']. With a view to identification, the S-center ESR characteristics have been mapped in detail. Its susceptibility is found nearly paramagnetic-Curie-Weiss type with critical temperature T-c=1.3+/-0.4 K, indicative of a weak ferromagnetic coupling; the defects appear clustered. Oxide etch-back experiments reveal that during degradation the oxide undergoes significant modification, dependent on depth into the oxide film. As to defect distribution, for T-an=1200 degreesC, the etch-back experiments show the S centers to predominantly occur near the oxide borders, with a sharp pileup within similar to40 Angstrom of the Si/SiO2 interface, and a more stretched out one (similar to150 Angstrom) towards the top surface; S and E-gamma' centers generally occur in anticorrelation. The S defects are susceptible to passivation in molecular H-2. From the salient ESR properties, the S center is suggested to be of the type SinO3-n=Si. (n=1,2). Though tentative, the observed weak hyperfine structure may be compatible with either the single n=1 defect or an overlap of both the n=1,2 types, the defect system exhibiting substantial randomness-induced variation in defect morphology. Based on the known interfacial SiO2 reduction process, the thermal degradation of the oxide as a whole is interpreted as effectuated by interface-released SiO.