A procedure has been developed to measure the relative concentrations of N2, O2, Ar, the Δ18O of O2, and the Δ15N of N2 trapped in Antarctic and Greenland ice cores. To date, we have analyzed 13 samples of trapped air in recent (<2200 years B.P.) ice from eight sites. The composition of the trapped gases was determined by mass spectrometry and the results expressed in delta notation relative to the present-day atmosphere. The average Δ15N and N2 and Δ18O of O2 in samples from the various ice cores ranged from +0.05 to +0.41 %o and from +0.18 to +1.1 %o, respectively. The heavy isotope enrichments are apparently due to gravitational settling accompanying diffusive equilibrium between the base of the firn and the overlying atmosphere. The ΔO2/Ar and ΔN2/Ar values ranged from -9.2 to +1.0 %o and from -4.8 to +4.6 %o, respectively. The differences between the elemental composition of ice core trapped gases and air result from gravitational fractionation, as well as separation of O2 and Ar relative to N2 during either bubble close-off or retrieval and storage of the ice cores. ¿ American Geophysical Union 1989