The solubility isotherms of the quarternary system Na+K+Cl−H2PO4−H2O are given for O° 25° and 75°. Additional data have been used for the establishment of the polytherms, between 0° and 100°, of the two «3 salt lines» (saturation with NaCl + KCl + KH2PO4, and NaCl + KCl + a sodium dihydrogenophosphate) and of two «2 salt surfaces» (saturation with NaCl + KCl, and NaCl + KH2PO4). The parameters of the two invariant points of the system have been established by interpolation. The salts NaCl and KH2PO4 form the «stable salt pair» at all temperatures between 0° and 100°.