Authigenic dolomite layers occur in the organic matter-rich sediments of Kau Bay, Indonesia. Dolomite formation is related to the anaerobic decomposition of organic matter which causes depletion of pore-water sulfate and increase in interstitial water alkalinity. X-ray diffractograms of Kau Bay dolomites show sharply defined peaks, including the superstructure reflections. Carbon isotopes indicate that a significant portion of the carbon is derived from the degradation of organic matter. The pore waters are highly supersaturated with dolomite, but at greater depths, pore waters are undersaturated or are near saturation with calcite and aragonite. Dolomite seems to have formed by direct precipitation from solution. The formation of dolomite is believed to be initiated by the high alkalinity levels found in the zone of anoxic methane oxidation. The rate of dolomite formation is controlled by the supply of calcium and is on the order of a few centimetres per 1 ka.