Observations made from Kitt Peak, AZ, of the 233.9 GHz emission line of 18O16O in the upper stratosphere and lower mesosphere are reported. A good model fit to the line is obtained by incorporation of pressure and temperature broadening effects, as well as a Zeeman splitting algorithm that uses a standard geomagnetic field model and a paramagnetic Hamiltonian description of the molecular energy states. These observations are used, along with the well known 18O16O mixing ratio, to establish absolute calibration for observations of other chemical species from Kitt Peak. Repeated measurements show no change in this absolute calibration between observation dates. The wide magnetic splitting (¿1.8 MHz) exhibited by this line with only six Zeeman components provides a unique test of middle atmosphere Zeeman effect model calculations, supporting the use of O2 lines by microwave atmospheric sounders to measure pressure and temperature.¿ 1997 American Geophysical Union