Impact experiments on water ice targets were carried out to investigate the partitioning of energy and the mass of water vaporized. A copper projectile accelerated by an electromagnetic gun impacted a low-temperature ice target under a high vacuum condition. The mass of vaporized water gas was measured as a function of impact velocity (54--329 m/s) and ice temperature (130--185 K). Some 0.01--0.03% of the impact energy is partitioned into ice vaporization at velocities ≤100 m/s. For impacts in the 100--180 m/s range, the energy partitioned into water vaporization drastically increases to 18--26% of the impact energy. The estimated shock pressure is 277 MPa for a copper-ice impact at 100 m/s. This is close to the pressure at the Hugoniot elastic limit for ice. It is considered that the drastic change in energy partition at 100 m/s is due to the large increases in surface area and localized heating via shear banding occurring at the Hugoniot elastic limit. ¿ 1998 American Geophysical Union