Mutual solubility of potassium vanadate(V) and iron(III) vanadate(V) was measured over a temperature range of (293.15 to 323.15) K. These results were used for the construction of a polytherm section in the KVO3 + Fe(VO3)(3) + H2O system. Additionally, the KVO3 influence on the Fe(VO3)(3) solubility and the solution density and viscosity interdependence on solution composition were determined. The paper presents also the approximation of the experimental data on the mutual component solubility for this ternary system with the use of mathematical functions. The data are essential in the determination of the optimum operating parameters for the selective precipitation of vanadium compounds from a postleaching solution of the spent vanadium catalyst.