Observations of stratospheric NO, NO2, and O3 from the Halogen Occultation Experiment (HALOE) are examined over the southern midlatitudes (35¿--45 ¿S) in the summers following the eruption of Mount Pinatubo, from early 1992 through mid-1995. The tracers HF and CH4, also observed by HALOE, are used to distinguish between the effects of chemistry and those of transport processes on the distribution of total reactive nitrogen (NOy) and ozone. Results show that at 17 mbar (about 28 km), in parcels of similar photochemical age, the abundances of summertime NO+NO2(NOx) increased dramatically between 1992 and 1993 and then leveled off in subsequent years. The 1992--1993 increase is coincident in time with a sharp drop in aerosol surface area density as the Pinatubo aerosol cleared the atmosphere and gives evidence of the sensitivity of NOx to sulfate aerosol even at these altitudes, where the cycling of NOy through HNO3 is rapid. Results also show that summertime ozone abundances at 17 mbar declined by nearly 10% between 1992 and 1993 and then, like NOx, remained about constant for the next three summers. The trend in ozone is opposite in sign to that observed at lower altitudes in the aftermath of the Pinatubo eruption and demonstrates the importance of the NOx catalytic cycles in the ozone loss budget above 26 km.¿ 1997 American Geophysical Union