The oxygen isotope fractionation between the structural carbonate of inorganically precipitated hydroxyapatite (HAP) and water was determined in the range 10–37 °C. Values of 1000 ln α(CO32- –H2O) are linearly correlated with inverse temperature (K) according to the following equation: 1000 ln α(CO32- –H2O) = 25.19 (±0.53)-T-1 - 56.47 (±1.81) (R2 = 0.998). This fractionation equation has a slightly steeper slope than those already established between calcite and water (O'Neil et al., 1969; Kim and O'Neil, 1997) even though measured fractionations are of comparable amplitude in the temperature range of these experimental studies. It is consequently observed that the oxygen isotope fractionation between apatite carbonate and phosphate increases from about 7.5 pour mille up to 9.1pour mille with decreasing temperature from 37 °C to 10 °C. A compilation of δ18O values of both phosphate and carbonate from modern mammal teeth and bones confirms that both variables are linearly correlated, despite a significant scattering up to 3.5 pour mille, with a slope close to 1 and an intercept corresponding to a 1000 ln α(CO32- –PO43-) value of 8.1 pour mille. This apparent fractionation factor is slightly higher or close to the fractionation factor expected to be in the range 7–8 pour mille at the body temperature of mammals.