A first-year sea-ice model, consisting of the dielectric properties and the geometry of ice and snow cover, has been used for interpreting brightness-temperature measurements of low-salinity sea ice. Dielectric measurements were made on about 400 ice samples, the salinity of which ranged from 0·1 to 1·5‰ by weight and the temperature from −18 to −01°C. Measurements were made with the electric field at angles of 0, 30 and 90° to the horizontal in the natural ice field. The effect of the fluctuations in the properties of ice and snow within the radiometer antenna beam has been included in the theoretical brightness-temperature model.