We present data on the relative concentration and isotopic composition of O2, N2, and Ar in gases trapped in bubbles of polar ice. O2/N2 and Ar/N2 ratios are generally lower than the atmospheric values, with O2 depletions greater than those of Ar. In samples whose O2/Ar/N2 ratios are highly anomalous with respect to modern air, Δ18O of O2 may be enriched by 0.1--0.2?. There is no indication that Δ15N of N2 is enriched by gas loss processes in any of the samples we have studied. Our data suggest that anomalies in the composition of air in bubbles with respect to the modern atmosphere are due to gas loss from ice during and after coring by a process which fractionates elements according to their molecular diameter (Craig et al., 1988) rather than according to their diffusivity. ¿ American Geophysical Union 1995