Atmospheric gases are trapped in ice sheets. These gases stored in air-bubbles at shallower depth are gradually transformed into clathrate hydrates below the depth where the hydrostatic pressure exceeds the dissociation pressure of the clathrate hydrates. We measured Raman spectra of air-bubbles and clathrate hydrates in Vostok Antarctic ice cores in order to determine the fractionation effects on the concentrations of gases during their transition process. The results showed variations of the N2/O2 ratios with depth. The average N2/O2 ratio in the air-bubbles increases from the atmospheric value at the beginning of the transition to 11.7 at the end. The average N2/O2 ratio for the clathrate hydrates is 2.0 at the beginning, and asymptotically approaches the atmospheric value. This fractionation is attributed to faster diffusion of O2 than N2 through the ice lattice. ¿ 1999 American Geophysical Union