Sea ice thickness information is important for seaice modelling and ship operations. Here a method to detectthe thickness of sea ice up to 50 cm during the freeze-up sea-son based on high incidence angle observations of the SoilMoisture and Ocean Salinity (SMOS) satellite working at1.4 GHz is suggested. By comparison of thermodynamic icegrowth data with SMOS brightness temperatures, a high cor-relation to intensity and an anticorrelation to the differencebetween vertically and horizontally polarised brightness tem-peratures at incidence angles between 40 and 50◦are foundand used to develop an empirical retrieval algorithm sensitiveto thin sea ice up to 50 cm thickness. The algorithm showshigh correlation with ice thickness data from airborne mea-surements and reasonable ice thickness patterns for the Arc-tic freeze-up period.