Significant amounts of SO42−, Na+, and OH− are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO42−. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na+, but very low concentrations of SO42−. The SO42− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na+ and SO42− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO42− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO42− is the same as that of aragonite. Na+ appears to have very little effect on the solubility product of calcites. The amounts of Na+ and SO42− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient (D">D) of SO42− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k0+ k1R">D = k0+ k1R where k0">k0 and k1">k1 are constants equal to 6.16 × 10−6">6.16 × 10−6 and 3.941 × 10−6">3.941 × 10−6, respectively, and R">R is the rate of crystal growth of calcite in mg·min−1·g−1 of seed. The data on Na+ are consistent with the hypothesis that a significant amount of Na+ occupies interstitial positions in the calcite structure. The distribution of Na+ follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na+ distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na+ contents of calcites are not very accurate indicators of environmental salinities.