Data on the carbon and oxygen isotopic composition of the carbonate sediments, limestones and calcite cements are compiled and reviewed, and an elementary exposition of the factors that control them is given. Typical compositions and diagenetic trends are displayed on δO18–δC13 scatter diagrams, and an attempt is made at an isotopic categorization of environments of carbonate lithification. Many limestones pass through several diagenetic environments, which are recorded isotopically by cement generations of distinct isotopic composition. Most limestones contain essentially 'marine' carbon, but extreme compositions can result from organic reactions, especially those involving methane; some of these may be important in petroleum exploration, but their volumetric abundance is probably quite small.

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