Potassium dihydrogen phosphate is widely used in industrial and agri- culture. The solvent extraction method for producing KH2PO4 from the more cheap extraction phosphoric acid and potassium chloride has gained growing attention recently. To evaluation influence SO42– on the crystallization KH2PO4 it is nessesary comprehensive knowledge of the phase equilibria in the system K+// H2PO42–, SO42– – H2O. The ternary system KH2PO4 – K2SO4– H2O was determined at 25 °C by the improved isothermal method of section. The heart of the method lies in determining the position of two fixed points on each limiting node that intersect the coordinate system at the composition of equilibrium solid phases. This allowed us to calculate the composition of nonvariant solution and established the composition of the solid phase equilibrium to nonvariant solution. The eutonic solution (% wt) was determined: 16.51, KH2PO4; 5.72, K2SO4; 77.77, H2O. Equilibrium solid phases saturating nonvariant solution were determined without isolating them from the liquid phase. The crystallization field of potas- sium sulfate and potassium dihydrogen phosphate were determined. The phase diagram was constructed based on the experimental results. The crystallization region of K2SO4 is the largest, while that of KH2PO4 is the smallest.