The geochemical evolution of the superficial brines in the 'Salar de Llamara' saline system is studied in this paper. Systematic sampling of saline solutions and sediments has pointed out the continuous equilibrium state of brines with respect to calcite and gypsum along the evaporative path. The high evaporation rate causes a progressive increasing in the concentration of brines, up to ionic strengths near 6 molal. From the mineralogical analysis of solid saline samples and the physicochemical treatment of solutions, a process of glauberite precipitation has been identified during the final stages. The formation of glauberite is associated with the partial simultaneous dissolution of the previously precipitated gypsum. The thermodynamic viability of this process has also been confirmed through the theoretical simulation of the evaporation process performed with the geochemical code PHRQPITZ.