This paper presents an analysis of odd nitrogen partitioning determined from instruments aboard the Upper Atmosphere Research Satellite (UARS). The UARS data set provides the first global maps and the best temporal coverage to date of NOx/NOy. Our analysis spans latitudes from 60 ¿S to 60 ¿N and altitudes between 22 and 35 km for three separate, month-long periods between August 1992 and March 1993. These periods are characterized by substantial stratospheric aerosol loading as a result of the eruption of Mount Pinatubo in June 1991. Active odd nitrogen (NOx) is estimated from sunset measurements of NO and NO2 made by the halogen occultation experiment (HALOE). Total odd nitrogen (NOy) is the sum of NOx and correlated nighttime measurements of ClONO2 and HNO3 made by the cryogenic limb array etalon spectrometer (CLAES). Typical values of UARS NOx/NOy during our study periods are 0.85 at 1000 K (35 km), 0.60 at 800 K (31 km), 0.25 at 650 K (26 km), and 0.10 at 550 K (22 km). Significant latitudinal gradients in nitrogen partitioning are observed in the winter hemisphere at altitudes below 1000 K, with the highest values of NOx/NOy found at midlatitudes. Comparison of the UARS NOx/NOy with predictions from a constrained version of the Goddard two-dimensional fixed circulation model of stratospheric photochemistry demonstrates generally good agreement at the altitudes and latitudes studied. The model estimates have a slightly low bias that diminishes as the aerosol loading decreases between August 1992 and March 1993. At lower altitudes under high aerosol loading, the model does not predict the winter hemispheric, subtropical gradients of NOx/NOy seen in the UARS data. This discrepancy may be due to the lack of similar gradients in the ozone and surface area density fields which constrain the model.¿ 1997 American Geophysical Union