The transformation from air bubbles to air clathrate hydrate crystals (air-hydrates) in polar ice sheets has been studied using a natural ice-core drilled at Mizuho Station. East Antarctica. In situ observations using an optical microscope were done under hydrostatic pressures of 14.7 - 22.5 MPa at 258 - 268 K. The air-hydrate nucleated only at the interface between an air bubble and ice from 20 to 330h after the pressurization. The nucleation ratio, which we defined as the number of air bubbles containing hydrate 330 h after pressurization divided by the initial number of air bubbles in a given volume, depended oil the excess pressure above the equilibrium pressure at a given temperature. Of two ice samples studied, this nucleation ratio was higher in the sample with greater chemical impurity content. Based oil the experimental results, we discuss the nucleation mechanism of air-hydrate caused by chemical impurity in polar ice sheets. (C) 2004 Elsevier B.V. All rights reserved.