An experimental model of chemical stratification in certain saline lakes by hydrate deposition and hydrate‐anhydrate phase transition was developed for the system sodium carbonate‐sodium sulfate‐sodium chloride. Mirabilite (Na2SO4• 10H2O) was the test hydrate. A population of an halotolerantEctothiorhodospirafrom Owens Lake, Calif., partitioned between brine and mirabilite upon decreased temperature, survived 100 days dark storage at 15°C equally well in the resulting liquid phase and in the mirabilite. The entrapped organisms did not survive the phase transition of mirabilite to thenardite, the anhydrate. Since the optimum water activity (a10) for this organism was 0.95, those remaining in the liquid phase benefitted by an increase ina10from 0.93 in the initial brine to 0.95 upon deposition of the hydrate. The “blebs of microorganisms”; reported to be included in hydrates of ancient Searles Lake, Calif., facies may have been formed contemporaneously with deposition of the salts. Implications for biogeochemical cycles of carbon and sulfur are discussed.