Measurements of the 18O/16O isotopic ratio of atmospheric carbon dioxide made by the University of Colorado, Boulder on flask samples provided by the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration appear to have been contaminated by oxygen isotope exchange between CO2 and water inside the glass flasks. The 18O/16O ratios measured for samples from high-humidity sites collected during 1990--1993 show considerable scatter and are believed not to represent the true oxygen isotopic composition of atmospheric CO2. Laboratory experiments have demonstrated that oxygen isotope exchange occurs between CO2 and water condensed on the flask walls. Complete isotopic equilibrium is not observed, even for storage times greater than 1 month; the extent of exchange differs for individual flasks. The experiments show that air samples to be stored in glass flasks must be dried to a dew point of 2 ¿C or lower before storage to eliminate artifacts associated with oxygen exchange. ¿ American Geophysical Union 1996