The calcium isotope ratios (δ44Ca = [(44Ca/ 40Ca)sample/(44Ca/40Ca) standard-1] · 1000) of Orbulina universa and of inorganically precipitated aragonite are positively correlated to temperature. The slopes of 0.019 and 0.015‰ °C-1, respectively, are a factor of 13 and 16 times smaller than the previously determined fractionation from a second foraminifera, Globigerinoides sacculifer, having a slope of about 0.24‰ °C-1. The observation that δ44Ca is positively correlated to temperature is opposite in sign to the oxygen isotopic fractionation (δ 18O) in calcium carbonate (CaCO3). These observations are explained by a model which considers that Ca2+ -ions forming ionic bonds are affected by kinetic fractionation only, whereas covalently bound atoms like oxygen are affected by kinetic and equilibrium fractionation. From thermodynamic consideration of kinetic isotope fractionation, it can be shown that the slope of the enrichment factor α(T) is mass-dependent. However, for O. universa and the inorganic precipitates, the calculated mass of about 520 ∓ 60 and 640 ∓ 70 amu (atomic mass units) is not compatible with the expected ion mass for 40Ca and 44Ca. To reconcile this discrepancy, we propose that Ca diffusion and δ44Ca isotope fractionation at liquid/solid transitions involves Ca2+ -aquocomplexes (Ca[H2O]n2+ · mH2O) rather than pure Ca2+-ion diffusion. From our measurements we calculate that such a hypothesized Ca2+ -aquocomplex correlates to a hydration number of up to 25 water molecules (490 amu). For O. universa we propose that their biologically mediated Ca isotope fractionation resembles fractionation during inorganic precipitation of CaCO3 in seawater. To explain the different Ca isotope fractionation in O. universa and in G. sacculifer, we suggest that the latter species actively dehydrates the Ca2+-aquocomplex before calcification takes place. The very different temperature response of Ca isotopes in the two species suggests that the use of δ44Ca as a temperature proxy will require careful study of species effects. © 2003 Elsevier Science Ltd.