According to the compositions of the underground gasfield brines in the west of Sichuan Basin, in China, solid-liquid equilibria in the quaternary systems KBr-K2SO4-K2B4O7-H2O and NaBr-Na2SO4-Na2B4O7-H2O at 298 K were measured by isothermal solution saturation method. The solubilities of salts and the densities of saturated solutions in these quaternary systems were determined. Using the experimental data, phase diagrams, water content diagrams, and solution density diagrams were constructed. The two phase diagrams were simple cosaturation type without complex salt and solid solution. The phase diagram of the quaternary system KBr-K2SO4-K2B4O7-H2O has one invariant point, three univariant curves as the boundary of KBr, K2SO4 and K2B4O7 center dot 4H(2)O crystallization fields. The quaternary system NaBr-Na2SO4-Na2B4O7-H2O at 298 K consisted of five univariant curves, two invariant points and four crystallization regions of Na2B4O7 center dot 10H(2)O, Na2SO4, Na2SO4 center dot 10H(2)O, and NaBr center dot 2H(2)O.