Kinetic isotope fractionation of 13C during precipitation of CaCO 3 under open system conditions has been investigated. The isotope enrichment factor εHCO3-- CaCO3 varies between -0.35 ± 0.23 and -3.37 ± 0.36%. at 25°C depending on the rate of precipitation and mineralogy, the enrichment of 13C in the solid carbonate phase decreasing with increasing precipitation rate. An estimate of equilibrium εHCO3- Calcite of between -1.83 ± 0.32 and -2.26 ± 0.31%. is calculated from slow precipitation runs. A surface diffusion crystal growth model is used to describe the combination of kinetic isotope effects on thermodynamic isotope fractionation during rapid diffusion controlled crystal growth. Under slow precipitation conditions εCalcite- Aragonite was estimated as -1.4%.; however, during rapid precipitation this fractionation appears to diminish and aragonite becomes less enriched in 13C.