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The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions refl ect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m area in Harrison’s Cave, Barbados, West Indies. Only some of the plate δC values and none of the plate δO values correspond to equilibrium values, indicating signifi cant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects. On each plate, speleothem calcite was deposited with distinct δO and δC compositions that increase progressively away from the growth axis, with up to 6.6‰ C and 1.7‰ O enrichments. The positive δC versus δO trends are likely a result of O and C Rayleigh-distillation enrichment in the HCO 3 – reservoir owing to progressive CO 2 degassing and CaCO 3 precipitation. The magnitude of the δC versus δO slope is likely controlled by the extent to which CO 2 hydration-hydroxylation reactions buffer the oxygen isotope composition of the HCO 3 – reservoir during calcite precipitation. Complete oxygen isotopic buffering of the HCO 3 – reservoir by CO 2 hydration-hydroxylation reactions will produce a vertical δC versus δO slope in calcite sampled along a growth layer. As oxygen isotope buffering of the HCO 3 –