Zeeman splitting of O2 molecular states in the Earth's upper atmosphere leads to polarized emission spectra. A 61 GHz radiometer operated as part of the Millimeter-wave Atmospheric Sounder (MAS), a core payload instrument of the NASA Space Shuttle ATLAS missions, observed such emissions. This instrument's high resolution spectrometer (200 kHz) allows us to verify for the first time Zeeman effect model calculations for the upper atmosphere in some detail. The results suggest some interesting new aspects for the research of the upper atmosphere. ¿ American Geophysical Union 1996