Microwave brightness temperatures of new, young, and optically opaque sea ice grown in a large tank were obtained in the course of a joint microwave experiment at CRREL in Hanover, New Hampshire, during the winters of 1983-1984 and 1984-1985. Dual-polarized observations were taken at frequencies of 10, 18, 37, and 90 GHz over a range of incidence angles, and the concurrent temperature and ice thickness were obtained. Bulk salinities as well as radar and dielectric properties were also measured concurrently by other investigators. Emissivity and degree of polarization were observed in detail during the early stages of ice growth and variations were found indicating that the ice became optically opaque at 10 GHz for ice thickness between 30 and 50 mm. The addition of a snow cover reduced the brightness temperature at the higher frequencies with little effect at 10 GHz. Artificial roughening of the surface reduced the degree of polarization considerably but changed the emissivity at vertical polarization only slightly. Cluster plots of the data shown six distinguishable surface types: optically opaque bare ice, thin ice (less than 15 mm), roughened ice, ridged ice, rotting wet ice, and open water.