To deconvolve the effect of growth rate and temperature on the boron partitioning into calcite and its isotope fractionation, seeded calcite precipitation experiments were performed at a constant temperature and various growth rates and at a constant growth rate and various temperatures. We show that boron partitioning increases with increasing growth rate and decreases with increasing temperature. The B isotope fractionation between calcite and B(OH)4- increases with increasing growth rate favoring the lighter B isotope for incorporation into calcite whereas no effect of temperature was observed within the temperature range investigated (12°C to 32°C). At the lowest temperature and growth rate δ11B of the calcite almost equals that of B(OH)4- in solution. Applying the surface entrapment model (SEMO) of Watson and Liang (1995) to our data, we demonstrate that the observed effects of temperature and growth rate on B concentration can be explained by processes in the near surface layer of the calcite crystal.