This paper investigates diurnal variations in the dynamical and chemical structure of the middle atmosphere observed by UARS. Dynamical variations are of tidal nature and appear prominently in the temperature field near the equator. The temperature field is smooth enough for higher harmonics of the diurnal cycle, which are unresolved by the UARS sampling, to be negligible. Diurnal variations in the temperature field can therefore be mapped synoptically. Daily maps of the diurnal variation of temperature exhibit meridional and vertical structure typical of the propagating solar diurnal tide. Through advective and photochemical effects, diurnal variations of temperature introduce diurnal variations of ozone in the middle and upper stratosphere, chemical variations which are mapped in similar fashion. Diurnal variations of other chemical species contain higher harmonics, which are undersampled by UARS, preventing diurnal variations in those species from being mapped reliably. Instead, the average diurnal variation over a month or longer is composited from observations at individual local times. In the middle stratosphere, diurnal variations of NOx are strongly influenced by the zonal mean distribution of NOx, which in turn is controlled by the mean meridional circulation. So too are diurnal variations of ClOx but with sign opposite to those of NOx, owing to the coupling of those families. ¿ 1999 American Geophysical Union