Abstract: The HAMSTRAD microwave instrument operates at 60 and 183 GHz and measures temperature and water vapor, respectively, from 0- to 10-km altitude with a time resolution of 7 min. The radiometer has been successfully deployed at Dome C (Concordia Station), Antarctica ($75^{circ}06^{prime} hbox{S}$, $123^{circ}21^{prime} hbox{E}$, 3233 m amsl) during the first summertime campaign for 12 days in January–February 2009. The radiometer has been continuously running since January 2010, hosted within a dedicated shelter. We have used the very first set of HAMSTRAD data, recorded when the instrument was outdoors, to assess its potential to sound the troposphere over Dome C, from the planetary boundary layer (PBL) up to the tropopause ($sim$6 km above surface, $sim$9 km amsl). We have compared the HAMSTRAD measurements to several sets of measurements performed at the Dome-C station or in its vicinity: meteorological radiosondes, in situ PT100 and Humicap sondes along the vertical extent of a 45-m tower, meteorological sensor attached to the HAMSTRAD instrument, and the spaceborne Infrared Atmospheric Sounding Interferometer (IASI) instrument onboard the EUMETSAT MetOp-A satellite in polar orbit. The variability of integrated water vapor (IWV) observed by HAMSTRAD with extremely low values of 0.5 $hbox{kg} cdot hbox{m}^{-2}$ was also measured by the radiosondes (very high HAMSTRAD versus radiosonde correlation of 0.98), whereas IASI cloud-free measurements did not reproduce well the HAMSTRAD IWV variation (weak HAMSTRAD versus IASI correlation of 0.58). The measurements of absolute humidity $(hbox{H}{2}hbox{O})$ from HAMSTRAD at Dome C cover a large vertical extent from the surface to about 6 km above surface with a high sensitivity in the free troposphere. The strong diurnal v- riation of $hbox{H}{2}hbox{O}$ observed by the in situ sensors in the PBL is not well detected by the radiometer. In the free troposphere, the HAMSTRAD versus radiosonde $hbox{H}_{2}hbox{O}$ correlation can reach 0.8–0.9. Around the tropopause, HAMSTRAD shows the same variability as IASI and radiosondes but with a dry bias of 0.01 $hbox{g} cdot hbox{m}^{-3}$. HAMSTRAD tends to show a wetter atmosphere by 0.1–0.3 $hbox{g} cdot hbox{m}^{-3}$ compared with radiosondes from the surface to $sim$2-km altitude and a drier atmosphere above by $sim!! hbox{0.1} hbox{g} cdot hbox{m}^{-3}$. The sensitivity of the temperature profiles from HAMSTRAD is very high in the PBL and in the free troposphere but degrades around the tropopause. The strong diurnal signal measured above the surface by HAMSTRAD (3–6 K) is consistent with all the other in situ data sets. The temporal evolution over the 12-day period in the PBL is also consistent with all other data sets (radiosondes, IASI, in situ sondes, and meteorological sensors). In the free troposphere and around the tropopause, the HAMSTRAD temporal evolution is consistent with that observed by radiosondes and IASI, although a cold bias exists compared with IASI and radiosondes around the tropopause. For heights less than 4 km above surface, HAMSTRAD correlates very well with radiosondes and in situ sensors (correlation better than 0.8) but less well with IASI (0.4). Below the tropopause, the IASI and HAMSTRAD correlation reaches 0.9, whereas above the tropopause, the correlation of IASI and radiosondes with HAMSTRAD is rather low ($<$ 0.5). Throughout the 12-day period (except on January 23), in the lowermost troposphere for heights less than 500 m above surface, the HAMSTR