Mutual solubility of Na2SO4 and V2O5 in water at the temperature range (293 to 323) K was investigated by the method of isothermal saturation. The vanadate ion concentration was determined by a spectrophotometric analysis using 4-(2-pyridylazo)resorcinol, concentration of sulfate ions by a gravimetric method as BaSO4, and the sodium ion concentration was determined by an atomic absorption spectrometry method. On the basis of the results, the section of the solubility polytherm for the Na2SO4 + V2O5 + H2O system was plotted. These data can be used to assess the possibility to utilize vanadium(V) oxide from the vanadate catalyst used in the process Of SO2 to SO3 oxidation for the synthesis of Na2CO3 or K2CO3.