An algorithm is developed which uses two microwave radiometer channels to estimate quantitative fractions of first-year and multiyear sea ice types. The algorithm is applied to data obtained from satellite sensors, and the data trends are used to refine values of the emissivities. The algorithm was tested, and results were in reasonable agreement with visual observations, where mixtures of first-year sea ice and multiyear sea ice were known to coexist. However, on a synoptic basis the satellite estimates differ from visual and radar means of classifying ice that has survived at least one melt season (old ice). A possible explanation for the discrepancy is that the emissivity of sea ice changes over time periods longer than one melt season.