Kaolin (dickite and/or kaolinite) cement occurs locally in the Chalk Group (Upper Cretaceous-Lower Tertiary) in the North Sea and the Derbyhaven Beds (Lower Carboniferous) on the Isle of Man, United Kingdom. Stable-isotope and petrographic data indicate that the kaolin cements in both the Chalk Group and Derbyhaven Beds precipitated during late diagenesis in either marine or modified marine pore waters. Kaolin cements in both formations are associated with hydrocarbons, which suggests that organic complexing of aluminum may be responsible for precipitation of kaolin cement. Published experimental studies have demonstrated that the mobility of aluminum is greatly increased by complexing with organic acids. In such systems aluminum would be released and made available for clay-mineral precipitation when the organo-aluminum complexes are destabilized. Increased whole-rock d 13 C values in Chalk Group limestones from the Eldfisk Field, where kaolin cement was widespread, provides corroborating evidence for the late diagenetic decarboxylation of organic acids, which would have released complexed aluminum. The presence of kaolin cement may thus be an indicator of the former presence of organic-rich pore fluids in limestones and provide an additional tool for determination of the pore-water history of limestones and sedimentary basins.