Annealing-induced degradation of Si/SiO2 has been studied in noble gas ambients (He, Ne, Ar). The vacuum postoxidation annealing induced interface degradation in (111)Si/SiO2, previously identified as intense creation of interfacial Si dangling bond defects (P(b)s: Si-3=Si .) from similar to 640 degrees C on, is found remarkably impeded, inversely proportional to the gas atomic diameter: He fully blocks the process up to similar to 800 degrees C, Ne suppresses partially, while the effect of Ar is marginal. This is attendant with a significant reduction of oxide degradation, including suppression of the degradation of the insulating properties of ultrathin SiO2 layers and generation of hole trapping centers. The data support the degradation model based on interfacial SiO(g) release, showing that the degradation mechanism may just occur as the result of the existence of a typical interstice/channel structure in thermal SiO2. The observed process represents blocking of a chemical reaction in a physical way.