We measured the temperature-dependent lattice constant of air hydrates in deep ice cores from Vostok, Antarctica, using single-crystal X-ray diffraction. Thus the thermal expansion coefficient was determined, the first for a structure II clathrate hydrate containing small guest molecules. Despite their high dissociation pressures, measurements were done at atmospheric pressure because the air hydrates had stabilized during their original residence at high pressure. At 200 K, the lattice constant and thermal expansion coefficient were 17.20 Angstrom and 44 (+/-9) x 10(-6) K-1, respectively. This thermal expansion coefficient is approximately equal to that from tetrahydrofuran hydrate, another structure II hydrate, but the lattice constant is 0.3% smaller. Crystal structure refinement determined that the total cage occupancy by air molecules was 0.9.