The solubility isotherms were determined for the NH4NO3 + NaVO3 + NH4VO3 + NaNO3 + H2O system at 293 and 303 K. Based on these isotherms, the maximum yields have been determined for the conversion of ammonium nitrate to ammonium meta-vanadate. The location of the triple points P-1 and P-2 has been determined on the equilibrium plots prepared as the planar projection, according to the Janecke method. The results are necessary for determining the optimal conditions for utilization of post-filtration lye obtained from the soda-chloride-saltpeter (SCS) method of soda ash production.