The electron microprobe has been used successfully to investigate the MgCO 3 distribution within cements of Pleistocene carbonate skeletal grainstones from West Indies. The resultant MgCO 3 distributions are accounted for by a model involving precipitation of carbonate cement within one or more chemically discrete diagenetic environments (phreatic, early vadose, late vadose). The MgCO 3 content of carbonate cements precipitated within a particular diagnetic environment has also been estimated from the Mg (super 2+) /Ca (super 2+) ratio of the water in the environment and the distribution coefficient of Mg (super 2+) . These estimates are in good agreement with the microprobe data. In the rock samples under consideration, almost all cementation occurred during recrystallization of aragonite and high-Mg calcite to low-Mg calcite rock. MgCO 3 distribution patterns acquired during the recrystallization of samples now 240,000 to 700,000 years in age remain unaltered. Further application and refinement of the model may allow quantitative evaluation of the relative importance of various diagenetic environments in the cements of ancient rocks.