The solubilities of ammonium vanadate and ammonium nitrate in the NH4VO3-NH4NO3-H2O System were investigated at 293, 303, 313, and 323 K by the method of isothermal saturation of solutions. A section of the solubility isotherm for the investigated system was plotted based on the obtained data. Knowledge of that dependence is necessary for developing the new method of using the postfiltration liquor from the soda-chlorine-saltpeter (SCS) method of sodium carbonate production, based on an equilibrium plot for the quaternary system of pairs of the exchange salts NH4NO3-NaVO3-H2O.