In February and March 1992 stratospheric ClO and HCl over northern Scandinavia were observed with the submillimeter-wave atmospheric sounder (SUMAS), as part of the European Arctic Stratospheric Ozone Experiment (EASOE). SUMAS is operated on board the German research aircraft Falcon and observes thermal emission lines in the frequency range 620--650 GHz. In this paper the instrument design and techniques to retrieve volume mixing ratios (VMR) from the measurements will be discussed. Results from the EASOE campaign will be presented including a detailed error analysis. A comparison of the retrieved profiles in the lower stratosphere indicates a strong decrease of ClO abundances from February to March, whereas no corresponding increase in HCl is observed. ¿ American Geophysical Union 1995.