According to Oppenheimer and Master (1965) the biological activity of microorganisms in algal mats produces pH changes in sea water which parallel the photosynthetic cycle. A diurnal pH fluctuation from 7.4 to 9.2 resulted in the dissolution of quartz and precipitation of carbonate. In our study of two algal pools of the Red Sea we failed to observe comparable diurnal pH fluctuations. Likewise, in laboratory. experiments diurnal pH changes were not observable. However, changes in redox potential correlated with exposure to or absence of sunlight. Sunlight causes increasing Eh and darkness decreasing Eh, probably as a result of oxygen production during photosynthesis. CO 2 generation which in Oppenheimer and Master's experiment caused a pH shift is probably offset by H 2 S production in the pools which we studied. Lacking pH fluctuations, we could not duplicate the experiments of Oppenheimer and Master (1965) in dissolving quartz and precipitating carbonate. This study implies that among thick algal mats in a comparable hypersaline setting precipitation of stromatolites does not result from pH changes due to photosynthesis.