Measurements of brightness temperatures of sea ice were carried out during both the MIZEX field experiments in 1983, the first in Februrary in the Bering Sea and the second in June/July in the northern Greenland Sea. In the Bering Sea thin growing sea ice types from black ice to 400-mm-thick snow-covered floes were investigated. Brightness temperatures increased with ice thickness up to 100 mm from values of 100 K for open water to as high as 250 K (e= 0.97) for thick ice, and a moderate dependence on snow thickness was found. In the Greenland Sea thick first-year and multiyear ice types were studied. Brightness temperatures were quite variable depending on the daily melt-freeze cycle superimposed on the seasonal warming, ranging from near blackbody values for melting conditions to multiyear-like spectra when the surface layers refroze. The melt season was sufficiently advanced, however, that first-year and multiyear ice could not be differentiated radiometrically.