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A novel technique was used for the calculation of diffusion coefficients in the niobium carbides and nitrides prepared by reaction diffusion. The temperature ranges investigated were 1500 °C to 2100 °C for the Nb-C system and 1400 °C to 1800 °C for the Nb-N system. Three independent theoretical approaches were applied and their results are compared. In the metalloid-rich phases, the concentration-dependent diffusion coefficients were calculated from the concentration profiles; two models of layer growth were used to obtain the concentration-independent diffusion coefficients in all phases. It was found that the diffusion coefficient of carbon in δ-NbC1−xshows a decrease with increasing metalloid concentration, whereas the diffusivity of nitrogen in δ-NbN1−xis nearly independent of the nonmetal concentration. The concentration dependence of the carbon diffusion coefficients in δ-NbC1−xis a result of a lower activation energy of carbon diffusion in the substoichiometric δ-NbC1−xthan in the δ-NbC. On the contrary, the activation energy of nitrogen in δ-NbN1−xdoes not change with the nitrogen concentration. This behavior could be explained by the different occupancies of metal sublattices, which remain constant in δ-NbC1−xbut decrease with increasing nonmetal concentration in δ-NbN1−x.