This article presents the results of equilibrium tests regarding the development of a new waste-free technology for converting ammonium nitrate(V) to potassium nitrate(V) using potassium metavanadate. To establish the best conditions for disposing of postfiltration liquor from the soda-chlorine-saltpeter (SCS) production process, a detailed analysis of the isotherm in the five-component system KNO 3 + NH 4 NO 3 + KVO 3 + NH 4 VO 3 + H 2 O was required. Equilibrium tests were carried out by isothermal saturation of solutions method at temperatures of 293.15 and 313.15 K. The results obtained were used to plot solubility isotherms using the Janecke method. The maximum conversion efficiencies of ammonium nitrate(V) to ammonium metavanadate are 97.53% at T = 293.15 K and 98.69% at T = 313.15 K based on the data for the triple point P 1 . These results, along with literature data, were used to prepare a material balance for the proposed technological concept for the disposal of postfiltration liquor using the SCS method.