Bottom ablation in the marginal ice zone (MIZ) is a fundamental process that controls the ice edge position in both the winter and the summer. During the east and west Marginal Ice Zone Experiments (MIZEX West and MIZEX East) in 1983, periodic sonic distance measurements between the overlying ice and transducers fixed on masts below ice floes determined the bottom ablation. The melt rates for floes within 15 km of the ice edge ranged from near zero when the ice was in water at the freezing point to 0.5 m d-1 when the ice entered warm water, greater than 3 K to the south. Simultaneous velocity and temperature measurements at 2 m below the ice in MIZEX West and at 2 m and 10 m below the ice in MIZEX East, show the response of the melt rate to changing oceanographic conditions. For the first time, the combined oceanographic and ablation measurements allow the direct calculation of the bulk heat transfer coefficient Ch between the ice and the ocean at 2 m and 10 m below the ice. For the Bering Sea experiment, Ch was 8¿10-4, while for the Fram Strait experiment Ch varied from 2¿10-4 to 4¿10-4. Within each set of experiments, Ch remained nearly constant over a wide range of melt rates, relative ice water speeds, and seawater temperatures. ¿ American Geophysical Union 1987