A generalized boundary condition for self-interstitial diffusion in silicon is proposed by assuming interstitial flux continuity across the interface. In the case of thermal oxidation, the majority of generated interstitials at the interface flows into the oxide, while only a small fraction of the interstitials diffuse into bulk silicon causing oxidation-induced anomalous diffusion of dopants. The boundary condition together with a point-defect-based diffusion model, predicts several oxidation-related phenomena which cannot be explained without the quite sophisticated models reported to date. Those anomalous phenomena are oxidation-retarded diffusion of boron and phosphorus at high temperature, stripe-width independence of oxidation-enhanced diffusion of P and B, and oxidation-enhanced diffusion and oxidation-retarded diffusion of phosphorus during HCl oxidation. The generalized boundary condition is also shown to be applicable to thermal nitridation, in which interstitial generation, at the interface is neglected. Using the boundary condition, stripe-width dependence of nitridation-enhanced diffusion of antimony is demonstrated.>