The boron isotopic compositions and boron concentrations of selected modern marine carbonates were analysed by negative thermal ionization mass spectrometry with a 2σ reproducibility of standards and samples better than 0.7%. It was found that the boron isotopic compositions of modern marine carbonates fall within a relatively narrow range ( +22.1 ± 3%. relative to NBS SRM951 boric acid standard) for all samples (biogenic, nonbiogenic, warm water, cold water, aragonitic, or calcitic). This is about 17%. lighter than seawater. This indicates that mineralogy, temperature, and vital effects are not primary controls on the observed isotopic composition. The narrow range in isotopic composition is in contrast to the wide range in boron concentrations of these samples (10.9-71.4 ppm). The boron isotopic compositions and boron concentrations in modern marine carbonates are consistent with preferential adsorption of the isotopically lighter B(OH) 41- aqueous species over the B(OH) 3 species before incorporation into the carbonate site. Because there is a pH control on the availability of B(OH) 1-4, the result is a pH control on the boron abundance in the carbonates. Possible second-order biomineralogic controls on pH may account for boron concentrations in calcareous organisms that produce low pH microenvironments at the site of CaCO 3 precipitation.