Phase equilibria in the Rb3H(SO4)2-RbH2PO4-H2O system have been studied by the method of additives under isothermal conditions (at 25.0°C). The limits of the crystallization ranges of the compounds are preliminarily determined. Concentration conditions for the crystallization of Rb2(HSO4)(H2PO4) and Rb4(HSO4)3(H2PO4) compounds are found.