The equilibrium solubility studies of the NH4NO3 + NaVO3 + H2O system at (313 and 323) K have been performed. On the basis of the results, the equilibrium plots for the system in a planar projection have been obtained according to the Janecke method. The plots enable the determination of the optimal conditions for the conversion reaction of ammonium nitrate into sodium nitrate with the use of sodium metavanadate. The source of ammonium nitrate is the postfiltration lye from the soda production with the soda-chlorine-saltpeter (SCS) method. The conversion of NH4NO3 from the postfiltration lye into NaNO3 cancels the risk of explosion during the process of production of ammonium-sodium saltpeter.