An interstitial model is developed to quantitatively account for the anomalous effects which accompany the diffusion of phosphorus in silicon from high surface concentrations. Phosphorus is assumed to diffuse as both a positively charged (intrinsic diffusion) and an uncharged interstitial (high concentration case). Silicon is assumed to diffuse as both an uncharged (instrinsic) and a negatively charged interstitial (high phosphorus concentrations). The effect of internal fields on charged species is included. An excellent fit is found with published experimental data if a surface enhancement factor, analogous to that for the case of oxidation enhancement, is used. Flat-top diffusion is modelled by assuming the formation of a dislocation array, which alters the point defect equilibria.