The solubility isotherms were determined for the Na2SO4 + KVO3 + NaVO3 + K2SO4 + H2O system at (303.15 and 323.15) K. On the basis of these isotherms, the maximum yields have been determined for the conversion of sodium sulfate into metavanadate salt. The location of the P-1 and P-2 triple points 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 recovery of sodium metavanadate from leaching solution obtained from utilization process of spent vanadium catalyst using sodium hydroxide solution.