Measurement of the 18O/16O ratio in atmospheric CO based on mass spectrometry after its oxidation to CO2, which is applied to separate it from air, is evaluated. A convenient method to correct for the isotope effect of the addition of an oxygen atom by the oxidant to the original CO molecule is given, results of which confirm earlier data. Measurement of CO extracted from air containing varying amounts of CO derived from transportation and other man-made pollution sources reveals that the 18O/16O ratios of CO from these sources approach that of atmospheric oxygen. Analysis of upper tropospheric samples collected using aircraft flights between New Zealand and Antarctica allows a determination of the 18O/16O ratio of biomass burning derived CO after its transportation into the extratropical regions. The inferred Δ18O value of 4.5? would indicate that the kinetic fractionation in the removal of CO by OH during transport has decreased the 18O content by as much as 10?. Evaluation of the Δ18O values for samples collected in the stratosphere allows a determination of this fractionation, and a value for k16/k18 of 0.992 is derived, close to laboratory results. Further, by using a southern hemisphere CO budget it is possible to estimate that Δ18O of CO produced via the reaction CH4+OH has a value probably as low as 0?. ¿ 1997 American Geophysical Union