The aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O is one of the most important subsystems of the sulfate-type salt lake brine located in the Qinghai-Tibet Plateau. The phase equilibria of the aqueous quaternary system Li2SO4 + Na2SO4 + K2SO4 + H2O were investigated at 273.15 K using an isothermal evaporation method. A sodium and lithium sulfate double salt (3Na(2)SO(4)center dot Li2SO4 center dot 12H(2)O) and a potassium and lithium sulfate double salt (KLiSO4) are formed in this quaternary system at 273.15 K. There are three invariant points, seven univariant curves, and five crystallization fields in the phase diagram of this system. The five crystallization fields correspond to sodium sulfate decahydrate (Na2SO4 center dot 10H(2)O), potassium sulfate (K2SO4), lithium sulfate monohydrate (Li2SO4,H2O), and two double salts KLiSO4 and 3Na(2)SO(4)center dot Li2SO4 center dot 12H(2)O. The double-salt glaserite (3K(2)SO(4)center dot Na2SO4) is not formed in this system at research temperature. The three invariant points have different attributes, which invariants E-1 and E-2 are of incommensurate points, whereas invariant E-3 is of a commensurate point. The water content and the density of the equilibrated solution change regularly with the mass fraction composition of potassium sulfate.