The sunset/sunrise NOx ratios measured in 1994 by the Halogen Occultation Experiment (HALOE) onboard the Upper Atmosphere Research Satellite (UARS) peak at up to 2.8 in the tropics around 30 km and decrease to 1 at the stratopause. These ratios are interpreted by using a fully diurnal one-dimensional model, which considers gas phase reactive nitrogen chemistry, and a two-dimensional model, which includes a more complete set of chemical reactions. The diurnal cycle of nighttime N2O5 formation followed by daytime photolysis entirely accounts for the observed ratios above about 35 km. In the lower to middle stratosphere, additional reactions involving ClONO2 and HNO3 also appear to contribute to the observed diurnal variations in NOx. The limitations of a simple function of ozone, temperature, and length of night, which estimates diurnal variations in NOx based on gas phase reactive nitrogen chemistry, are tested through comparison to one-dimensional model results. The function approximates the one-dimensional model sunset/sunrise NOx ratios well in the lower stratosphere but overestimates them by up to 10% in the upper stratosphere above about 35 km, corresponding to a large overestimate of N2O5 formation. The function is extended to estimate sunrise N2O5 in the middle stratosphere. ¿ American Geophysical Union 1996