In this paper we derive equations for evaluating the supersaturation, beta, and the supersaturation rate, R-beta, for the drowning-out crystallisation under isothermal conditions and assuming a constant rate of the addition of the co-solvent, Such equations are applied to the drowning-out crystallisation of single and double sulphates in the M2SO4-LiSO4-H2O (M = Na, Rb) ternary systems. A linear relationship between In R-beta and In beta (max), similar to that reported For the crystallisation by cooling, evaporation and chemical reaction, has been found,