According to the composition of the brine resources in the west of Sichuan Basin

phase equilibria in the quaternary system Na+

K+//Br-

SO42--H2O at 373 K is measured by the isothermal solution saturation method

and the solubilities and densities of the solution are determined experimentally. Using the experimental results

the dry salt phase diagram

water diagram and the densities versus composition diagram are obtained. In the phase diagram of quaternary system Na+

K+//Br-

SO42--H2O at 373 K

the double salt Na2SO4·3K2SO4 is found. There are three invariant points

seven uninvariant curves and five crystallization fields in the quaternary system. The five crystallization fields correspond to NaBr

Na2SO4

K2SO4

KBr and Na2SO4·3K2SO4(Gla)

respectively. The crystallization field of NaBr has the smallest crystallization area

whereas the double salt Na2SO4·3K2SO4(Gla) has the biggest crystallization field in the quaternary system. It means that the double salt Na2SO4·3K2SO4(Gla) has the smaller solubility

and it can be easiest separated from solution. Compared with the two phase diagrams of quaternary system Na+

K+//Br-

SO42--H2O at 323 K and 373 K

the result shows that the numbers of invariant points

crystallization fields and unvariant curves are the same. The double salt all forms in the phase diagrams of quaternary systems at two different temperatures. But univariant curve GH of the corresponding balance solid phase is different and the crystal water of sodium bromide has disappeared at 373 K. In comparison with the quaternary system Na+

K+//Cl-

SO42--H2O and the quaternary system Na+

K+//Br-

SO42--H2O at 373 K

the two phase diagrams have very similar shapes

each of them having three invariant points

seven univariant curves and five crystallization fields. The crystallization field of the salt NaBr is apparently smaller than that of NaCl. It is also found that halide has the salting-out effect on sulfates. The water content and the density transformation rules are discussed simply. The water content is lower with the higher solution of bromine and the density is higher with the higher solution of bromine.