When discussing diffusion processes of dopants at very high concentrations, i.e., close to their solubility limit, it is absolutely necessary to take into account the fact that the number of substitutional sites accessible to additional donors/acceptors is decreasing or equal to zero when their concentration approaches/exceeds the solubility limit. It turns out that this results in a largely enhanced diffusion at dopant concentrations comparable with their solubility limit. Furthermore, for simple diffusion mechanisms it is possible to derive an analytical expression for the effective diffusion coefficient, depending on the dopant concentration, which can be used in process modeling.