Presented at the 14th Specialist Meeting on Microwave Radiometry and Remote Sensing of the EnvironmentPassive microwave measurements from the Soil Moisture Ocean Salinity (SMOS) mission, operating in the L-band at a frequency of 1.41 GHz, have been used to retrieve sea ice thickness with promising results, and a daily data product is now available from the Integrated Climate Data Center of the University of Hamburg. The purpose of this work is to apply a similar technique to measurements from two other L-band passive instruments, Aquarius and SMAP. Aquarius is a NASA sensor flown aboard the Argentinean SAC-D spacecraft and also operating at L-band. The instrument was designed for the main goal to measure sea surface salinity with a spatial resolution of 150 km, and it is composed by three radiometers, operating at the same frequency as SMOS, and one scatterometer. The Soil Moisture Active Passive (SMAP) is a NASA mission developed with the overall objective to monitor global soil moisture with unprecedented resolution, sensitivity, area coverage, and revisit time. It includes a conically-scanning passive radiometer operating at the same L-band frequency as SMOS and Aquarius, and an L-band (1.22 to 1.30 GHz) radar. The SMAP radiometer measures brightness temperatures at a constant incidence angle and spatial resolution of 40 km, while the radar has a spatial resolution of 3km. Aquarius and SMAP brightness temperature data are used to estimate sea ice thickness in the polar regions. The method is based on the inversion of a radiative transfer model for ice-covered sea. This model predicts the emission from ice-covered sea and is similar to the one used by the SMOS group. The sea ice thickness values retrieved from Aquarius and SMAP measurements using this technique are compared with the SMOS data products and also with estimates from Cryosat 2 radar altimeter . Since Aquarius ceased operation due to component failure on at the beginning of June 2015, while the SMAP radiometer began operating at the end of March 2015, data from April 2015 are used in the comparison. Results obtained using Aquarius and SMAP data are consistent with the SMOS sea ice thickness product. how a consistent behavior, but they have higher uncertainties due to both possible ancillary input inaccuracy and the less refined approach compared to the SMOS sea ice thickness algorithm. The effect on the sea ice thickness retrieval of the differences in operating principles, geometry and other system parameters between the instruments is discussed. Uncertainties due to the limited knowledge can also play an important role. The presence of melt ponds or snow over sea ice is also considered.