Thermal post-oxidation interface degradation in ͑111͒ Si/SiO 2 has been isolated by electron-spin resonance ͑ESR͒ as a permanent P b ͑SiwSi 3 ͒ interface defect creation. This process, initiating from ϳ640°C onward, reveals interface breakdown on an atomic scale as interfacial SiO bond rupture. The crucial creation step has been isolated as thermal cycling in an O-free ambient. Once created, the new P b system exhibits similar fully reversible H passivation-depassivation kinetics as the preexisting one, naturally introduced during oxidation. ESR is herewith raised to a powerful probe for Si/SiO 2 degradation.