Validation for adequacy description of the vapor-liquid phase equilibrium at the theoretical step of separation (TSS) was carried out in three stages. At the first stage, the calculated and known experimental data on the P_(H_2 O)^o and P_(D_2 O)^o vapor pressure of pure component and vapor pressure differences of pure components ∆p = pH2O – pD2O of the H2O and D2O components were compared. At the second stage, a comparison of the calculated and known experimental data on the separation coefficient α was considered. The relative deviation of the calculated values from the experimental values did not exceed 0.13%. At the third stage, the calculation of the phase equilibrium at the TSS was performed according to the algorithm developed by the authors, which included the calculation of the activity coefficients of the H2O and D2O components for a non-ideal mixture of H2O-D2O. The relative deviation of the calculated boiling point of the pure H2O and D2O components at a given pressure from the experimental one does not exceed 0.02% for H2O and 0.5% for D2O. The analysis of the obtained results indicates an adequate description of the vapor-liquid phase equilibrium at the TSS for the H2O-D2O mixture.