Mutual solubility of potassium metavanadate and potassium nitrate(V) was investigated in the KVO3-KNO3-H2O system at the temperature range 293.15-323.15 K. The equilibrium studies were conducted using isothermal saturation of a solution method. On the base of the received results, a fragment of the solubility polytherm for the investigated system was plotted. The knowledge of the course of that isotherm for the ternary system is necessary to plot the equilibrium diagram iii a planar projection prepared according to the Janecke method for the systems of exchange salts NH4NO3-KVO3-H2O and to determine optimal conditions for utilization of the postfiltration liquor from soda production with the soda-chlorine-saltpeter method.