[en] Software package UNIEXT has been developed for modelling the solvent extraction of radionuclides. The program for computing phase equilibria and distribution coefficient of the radionuclide in mixed-solvent systems is described. Application of the corresponding thermodynamic model to the calculations makes possible to predict properties of the extraction mixtures, which have not been investigated by chemical methods. A method allows building phase diagrams of the extraction mixture, fitting parameters of model from experimental database, and investigating solvent and ligand effects on separation process. Several examples of uranyl salts -water -organic solvents phase diagrams computed by UNIEXT are provided and compared with the experimental data. The UNIEXT package is especially useful in planning and executing chemical experiments aimed to find new efficient extraction systems for radionuclides. (author)