Southern Tunisian's natural brines represent an important source of mineral salts with a high economic value. These salts are recoverable by simple solar evaporation. Indeed, during isothermal solar evaporation it is possible to recover mineral salts and to determine the precipitation sequences of the different salts as a function of the composition and the density of the brine. In the present investigation the variation of the ionic composition of a low magnesium concentrated brine during isothermal evaporation was measured; then the experimental pathway of the point representing its composition on the oceanic quinary diagram Na+, K+, Mg-2+,Mg- Cl-, SO42- //H2O was plotted. In order to 4 understand the observed chemical equilibrium better and, especially, to predict the precipitation sequences of mineral salts, during solar evaporation at 25degreesC, the theoretical pathway of the same studied brine was calculated using Pitzer's ion interaction model.