Several years of ozone measurements from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are analyzed using a two-dimensional Fourier series in day of year and time of day. Because of limited temporal coverage near local noon, only the diurnal and semidiurnal components are included. Data are investigated in detail at 28 ¿N in the middle stratosphere to lower mesosphere, where the data are considered most reliable. The observations show that ozone is a maximum in the afternoon at 3 mbar and a minimum in the afternoon at 1 mbar and above with a narrow transition zone of reduced diurnal variation in between. This strong dependency on altitude in the transition from a maximum in the afternoon to a minimum in the afternoon, coupled with the small percentage changes in ozone, imposes strict requirements on the data and on the analysis of the data. Comparisons are made with results from a photochemical box model run at 11 levels between 0.46 mbar and 21.5 mbar for 28 ¿N at spring equinox and near the solstices. This is the first time that a data analysis and model comparison of this kind has been made, leading to the identification of relatively small diurnal variations, especially in the transition zone. In the middle stratosphere the model results are in poor agreement with the observations because of the influence of stratospheric dynamics which are neglected in the model runs. In the upper stratosphere the model shows the expected underprediction of absolute ozone amounts, although the percentage change from the midnight value is in excellent agreement with the observations and in particular correctly simulates the diurnal variation in the transition zone between 3 and 1 mbar. Model sensitivity studies are performed to determine the effects of major reaction rate changes and simplified tidal effects.¿ 1997 American Geophysical Union