The thermal analysis and X-ray powder diffraction studies of the Li2SO4-Na2SO4 system, including the high-temperature X-ray diffraction technique, have elucidated four phases of variable composition: three solid solutions based on the alpha-Li2SO4, alpha-Na2SO4, and alpha-LiNaSO4 high-temperature polymorphs, and a low-temperature beta-LiNaSO4 phase. alpha-Na2SO4-Base solid solution disintegrates into two phases via a monotectoid phase transformation. It is quite probable that the monotectoid process is related to the conversion of the second-order phase transition to the first-order phase transition.