Microwave (λ~1.2 mm) spectral emission line observations of HO2, O3, and H218O from the upper stratosphere and mesosphere (45--80 km) were obtained in April 1992, January 1993, and April and December 1996, using the Kitt Peak National Radio Astronomy Observatory (NRAO). Significant improvements to this, the only method for making mesospheric HO2 measurements, have been made with hardware upgrades allowing observation of a stronger emission line than described in previous work. Data analysis has been improved by (1) recalibration of the (primarily astronomical) instrument for atmospheric studies, (2) incorporation of updated laboratory spectroscopic work, and (3) use of an improved photochemical model. Diurnal HO2 data analyses employing 30 min to 2 hour observations show measured HO2 abundances at 50--80 km altitude are 23--47% higher than photochemical model values at mid day, agree with model values prior to 0900 local time (LT), and exceed model mixing ratios by 70--100% immediately after sunset. Coordinated diurnal measurements of mesospheric O3 find a model O3 deficit consistent with model underprediction of ozone seen in all comparable data sets. HO2 observations are critical to understanding mesospheric chemistry, in that simultaneous model underpredictions of this ozone-destroying species and of ozone place stringent constraints on proposed sources of model error. The ozone model deficit and 0900--1800 HO2 model deficit are simultaneously resolved with a 40% reduction in the rate coefficient k(HO2+O→OH+O2). We review other modeling studies of this rate change, indicating it has negligible effect in the lower stratosphere and improves a diverse set of model results from the upper stratosphere to mesopause. ¿ 1998 American Geophysical Union<