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) of SO₄²⁻ in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k₀+ k₁R where k₀ and k₁ are constants equal to 6.16 × 10⁻⁶ and 3.941 × 10⁻⁶, respectively, and 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.