Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO 2 degassing, passive CO 2 degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite–water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg 2+ ions is required to nucleate and precipitate aragonite from Na–Ca–Cl–HCO 3 solutions under these experimental conditions, the influence of the total Mg 2+ concentration (up to ∼0.9 molal) on the aragonite–water oxygen isotope fractionation was examined at 25 °C. No significant Mg 2+ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg 2+ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg 2+. On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite–water fractionation is proposed over the temperature range of 0–40 °C: 1000 ln α aragonite - water = 17.88 ± 0.13 ( 10 3 / T ) - 31.14 ± 0.46 where α aragonite–water is the fractionation factor between aragonite and water, and T is in kelvins. Given the analytical and statistical errors associated with this and previous determinations, the new relation reveals that many biogenic aragonites are precipitated at and or very near oxygen isotope equilibrium with their ambient water. When the new aragonite–water expression is combined with the calcite–water calibration published by Kim and O ’Neil [Kim S. -T., and O’Neil J. R. (1997) Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. Geochim. Cosmochim. Acta 61, 3461–3475], a positive aragonite–calcite fractionation (∼0.8‰ at 25 °C) is obtained over the temperature range investigated.