This paper integrates the degree of carbonate saturation into a general model for the origin of dolomite in organic-rich marine sediments. Degradation of organic matter by sulfate-reducing bacteria promotes early dolomite precipitation by simultaneously increasing the carbonate alkalinity and lowering the sulfate ion concentration to near zero. The interstitial water in organic-rich sediments of several Deep Sea Drilling Project sites where dolomite is believed to be precipitating is up to 103 times supersaturated with respect to near-ideal dolomite and up to 10 times supersaturated with respect to calcite. The highly supersaturated interstitial waters appear to overcome low temperature kinetic barriers to precipitate a nonideal, fine-grained calcian dolomite. The degree of supersaturation with respect to dolomite decreases, and the interstitial water approaches carbonate equilibrium with increasing burial depth at several of these sites. Removal of sulfate ion is important because sulfate ion may inhibit dolomite precipitation. A calcareous sediment is not required, but the presence of calcite or aragonite appears to greatly enhance dolomitization by providing an additional source of Ca and carbonate ions.