Solid-liquid equilibria in the quaternary system KCl-KBr-K2B4O7-H2O at 323 K were measured by isothermal solution saturation method. The equilibrium solid phases, solubilities of salts, and densities of saturated solutions in the system were determined. The experimental data were used to plot the phase diagram of the quaternary system KCl-KBr-K2B4O7-H2O at 323 K. It was found that the system contains solid solution K(Cl,Br), their solubility diagram has no invariant point, but has a univariant dissolution curve, and two crystallization regions corresponding to K(Cl,Br) and potassium borate quahydrate (K2B4O7 center dot 4H(2)O). Density values in the equilibrium solution increase with an increase of the potassium bromide concentration but decrease with an increase of the potassium chloride concentration. On the basis of the solubility data of salts and corresponding to the solid solution composition data for the quaternary system, the solid solution compositions were fitted with quadratic equations for the equilibrium liquid composition. The calculated results for the solid solution K(Cl,Br) are discussed briefly.