The equilibrium solubility studies of the Na2SO4 + KVO3 + H2O system at 293.15 and 313.15 K have been performed. Based on 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 potassium metavanadate into complex salt sodium, potassium metavanadate with the use of sodium sulfate. The aim of the process is the recovery of vanadium compounds from solutions obtained after leaching spent vanadium catalyst. (C) 2015 Elsevier BM. All rights reserved.