Phase equilibria in Na,K//SO4,CO3,HCO3-H2O system at 50°C are studied by the translation method. 32 divariant fields, 36 univariant curves, and 13 invariant points are determined in the system. The first closed phase diagram (phase complex) of the investigated quinary system is constructed. The constructed phase diagram is fragmented into divariant fields of co-crystallization of the two phases at the quinary level.