An oxidation-induced fixed oxide charge Qss is known to be generated in the SiO2-Si interfacial region. In the light of recent determinations of the composition and width of this interfacial transition region, a reexamination of the nature of this charge is undertaken. A model, consistent with known transition region composition, is proposed in which a positively charged silicon-oxygen complex is formed as an intermediate reaction product of the oxidation process. This intermediate may be trapped within the transition region when the oxidation reaction is terminated by quenching in the oxidizing ambient. The concentration of this complex is proportional both to the excess oxygen concentration and to the nonoxidized Si-Si bond density in the transition region. The model is also consistent with the dependence of Qss on such factors as oxidant pressure, oxidation temperature, inert-gas annealing, and substrate orientation. © 1978, The Electrochemical Society, Inc. All rights reserved.