The solubility of sodium vanadate(V) and sodium sulfate(VI) in the system NaVO3 + Na2SO4 + H2O within a temperature range from (293.15 to 323.15) K was measured with the method of isothermal saturation of solutions. The concentration of ions in solutions was determined using the EDXRF method. The part of the polythermal solubility surface for the investigated system was plotted based on the obtained data. Additionally, the solution density and viscosity was investigated. Knowledge of these data is necessary to evaluate the possibility of the use of vanadium compounds from a spent vanadium catalyst for the production of Na2CO3, and to assess the possibility of selective precipitation of vanadium compounds from a solution after leaching the waste.