Dolomite precipitation has been reported in sediments of marginal-marine hypersaline lagoons near Rio de Janeiro, Brazil, and has been related to microbial sulfate reduction. Our field study of pore-water and sediment geochemistry in two lagoons reveals only modest degrees of SO4 reduction and even H2S oxidation. Significant differences in dolomite contents, salinity, and carbon-sulfur cycling occur in the two lagoons; pore waters from the most dolomite-rich sediments exhibit significantly elevated SO4/Cl ratios relative to seawater. Moreover, the O isotope composition of SO4 in all lagoonal waters has evolved significantly from seawater values. This suggests active sulfur recycling via H2S oxidation, which provides acid that decreases pore-water saturation state for high-Mg calcite and aragonite, thermodynamically favoring the precipitation of dolomite. The open hydrogeochemical nature of these lagoons maintains SO4 concentrations below gypsum saturation, allowing dolomite to form from fluids with normal marine Mg/Ca.