Studies in Sabkha Faishakh on the west coast of Qatar in the early 1960s showed that much of the aragonite sabkha sediment has been converted to cryptocrystalline and microcrystalline dolomite in the last few thousand years under the influence of its interstitial, lagoon-derived brines whose Mg/Ca ratio was boosted by the precipitation of gypsum (Illing et al. 1965). Because of the comparatively simple hydrological system in the part of the sabkha studied, we have been able to show from chemical analyses of water samples collected along a 2-km line of pits across the sabkha that, as dolomitization proceeds, the brines lose magnesium and sulfate ions in a nearly one-to-one ratio in conformity. with the reaction 2CaCO 3 + Mg (super ++) + SO 4 (super --) -->CaMg(CO 3 ) 2 + CaSO 4 .2H 2 O The radical losses were determined by assuming that the chloride content of the brines changes solely as a function of evaporation. The correlation coefficient of magnesium vs. sulfate losses in 17 water samples analyzed is 0.98, and the slope of the line is 0.89. Changes in the bulk mineralogical composition of the sediment confirm the production of approximately equivalent amounts of dolomite and gypsum, as put forward by Klement nearly a hundred years ago.