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The fractionation of oxygen‐18 in the reaction of copper with oxygen of natural isotopic composition has been measured over the temperature range 68—256°C. The oxide films (estimated thicknesses varying from 150—2500 A) were removed for O18 assay by treatment with hydrogen at 350°C. Using the CO2 equilibration technique, the O18/O16 ratios for the resulting water samples were determined mass spectrometrically and compared with the ratio for a reference sample of water prepared from the original oxygen gas.The direction of the fractionation indicated that O216 reacts preferentially compared to O16O18. The isotope effect appeared to be independent of the oxygen pressure over the limited range studied (2—25 cm Hg). The magnitude of the fractionation factor was 2.0% at 150°C, with a small negative temperature coefficient. From this it was possible to estimate a value of ΔEact=17 cal/mole, which may be compared with the calculated isotopic zero‐point energy difference of 64 cal/mole. The theoretical implications of the experimental results are discussed

Oxygen-18 Isotope Effect in the Reaction of Oxygen with Copper