The ESRPbcenter has been observed in thermally oxidized single‐crystal silicon wafers, and compared with oxide fixed chargeQssand oxidation‐induced interface statesNst. ThePbcenter is found to be located near the interface on (111) wafers. Itsganisotropy is very similar to that of known bulk silicon defects having SiIIIbonded to three other Si atoms; thePbunpaired electron orbital, however, is exclusively oriented normal to the (111) surface. ThePbcenter cannot be identified with any other known defect in Si or SiO2; in particular, it is totally unlike the commonE′ center of SiO2. In contrast toQss, bothPbandNstwere found to be greatly reduced by steam oxidation and hydrogen annealing. BothPbandNstmay be regenerated by subsequentN2anneals at 500 °C. In a graded series of samples,PbandNstare found to be proportional and nearly equal in concentration. This possible confirmation of SiIIIat the interface, and correlation withNst, support the theoretical indication of an SiIIIband‐gap energy level. TheE′ center is unobservable, and if present, exists only in a concentration well below that ofQss. Thus, in addition to a lack of strong correlation withPb,Qssis evidently not due toE′ centers in their normal charge state. Overall, ESR is judged to be a useful technique for research on silicon wafer defects.