The purpose of this work was to construct a diagram for solubility and for activity of water in saturated solutions of the quaternary reciprocal system Na, NH//4 parallel H//2PO//4, SO//4H//2O at 25 degree from data on phase equilbria in the ternary subsystems, and to study experimentally the solubility diagram in order to verify the calculated results. It can be seen that the solubility diagram of the system Na, NH//4 parallel SO//4, H//2PO//4-H//2O at 25 degree includes six crystallization fields of solid phases, eight lines of three-phase equilibrium, and four invariant points, corresponding to equilibrium of the solution with three solid phases.