Abstract: The diffusion coefficients of Li⁺ and Na⁺ ions in glass and glass-forming melts of lithium and sodium niobium phosphates, which are in satisfactory agreement with the experimental data available in the literature, are obtained by the molecular dynamics method. The nonlinear change in the migration parameters (manifestation of the mixed alkali effect) is confirmed at the equivalent replacement of Li⁺ ions by Na⁺ ions in this system. The analysis of the radial pair correlation functions demonstrates an increased formation of lithium-sodium ionic pairs in bimetallic compositions, responsible for manifestation of the mixed alkali effect.