A study has been made of the heat budget just before and during the season of ablation, of first year sea ice near the head of Tanquary Fiord. Ablation started as the air temperature approached 0 °C, producing a decrease in albedo from approximately 0.6 to 0.2 in less than a week. Typical values of incident shortwave radiation were 800 cal cm−2 day−1 on clear days and 400 cal cm−2 day−1 during heavy overcast. The net influx of all-wave radiation was about 350 cal cm−2 day−1 during the ablation period, and resulted in a rate of ablation of ice of approximately 4 cm day−1.It is shown that the flux of radiative heat is the dominant factor determining the ablation rate and the change in thermal content of the ice sheet. Upward conduction from the sea is small, except when surface melt runoff occurs and collects in a stable layer immediately under the ice sheet. Even partial refreezing in this layer may release large quantities of latent heat to increase the rate of bottom conduction appreciably.