The natural waters of the Lakes Region in the Main Ethiopian Rift (MER) show serious problems of chemical quality. The high content of fluoride reaches 300 mgl(-1) and affects the health of the population who live in this area. Furthermore, the alkaline and sodic characteristics (8<pH<10 and 0.9 x 10(-4) < Na < 0.35 Ml(-1)) of these waters destroy the soil for agricultural use. This geochemical study is conducted using analytical data issued from literature (UNDP, 1973: Chernet, 1982) and several field sampling and laboratory analyses completed during the last 4 yr. Chemical analyses of 320 samples taken throughout the region have been analysed using the computer software AQUA for geostatistic, statistic, chemical equilibria and simulation of evaporative concentration process. The results show that the waters issuing from volcanic rocks are characterised by a positive alkalinity residual of calcite. When they concentrate due to the effect of climate, the precipitation of calcite causes a decrease in the chemical activity of calcium. This results in an increase in solubility of fluoride, previously controlled by equilibrium with CaF, and the element concentrates without being significantly affected by the precipitation of fluorite. As water concentrates, the low concentration of dissolved calcium emphasises the alkaline characteristics. As a consequence, the pH reaches very high values (9-10) which make the waters unsuitable for agriculture. The adverse elements. namely fluoride, sodium, and alkalinity accumulate in the lower zones of the basins. The acquisition of high fluoride content and the alkaline-sodic characteristics depends primarily on the unbalanced initial stage between the carbonate alkalinity and calcium [(HCO3) > 2(Ca + Mg)]. which results from the weathering and dissolution of the volcanic rocks. The predictive qualities of the model AQUA were tested in the context of this region. A simulation of the concentration of the waters by evaporation has shown chemical variations relatively similar to those obtained with field data which include the minor chemical elements such as the fluoride or the dissolved silica. For field data, enrichment is more variable and may represent localised conditions of mineralization. This model is then used to study a method of defluoridation by supplying these waters with calcium in the form of gypsum. This method also allows the reduction of the alkalinizing and sodifying characteristics of the waters without reaching the fluoride standard concentration accepted for drinking waters. For drinking purpose, waters of the Ethiopian rift should undergo another method of defluoridation.