Distribution coefficient data for the coprecipitation of Sr (super +2) with aragonite and calcite at low temperatures enable the Sr (super +2) concentrations of carbonate minerals and rocks to be interpreted in terms of the Sr/Ca ratio of the solution and the temperature. Sea water and other waters have been analyzed to provide the necessary solution data. Predicted Sr (super +2) concentrations of aragonite from sea water are 8290 + or - 850 ppm for the Bahamas, about 17 percent lower than actually found. Organic complexing of cations may account for the differences. Aragonite from Bahaman coral and algae shows that the Sr (super +2) uptake seems to be affected only slightly by biochemical fractionation, in comparison with most molluscan aragonites. The Sr (super +2) concentrations of diagenetically altered limestones are demonstrated to be of potential value in indicating the mechanism of diagenesis.