Measurements from the Halogen Occultation Experiment (HALOE) of the upper stratosphere show increases in HCl and H2O and decreases in CH4 and O3 during the period 1992--1995. These changes all coincide with the decline of solar cycle 22. Using a simple photochemical model, we find that 4 major components contribute to the O3 decrease: 1) an increase in total chlorine as indicated by increasing HCl 2) an additional increase in reactive chlorine due to repartitioning of Cly by the decreasing CH4 3) a decrease in odd oxygen production due to decreased solar flux and 4) an increase in odd hydrogen loss due to increasing H2O. At 2 mbar, the Cly repartitioning is the largest cause of O3 changes. Because the Cly repartitioning coincides with decreasing solar flux, some recent observational estimates of the long-term response of upper stratospheric O3 to solar UV irradiance variability may be too large. Compared with the HALOE O3 data, the model O3 exhibits a larger negative trend. This appears to be because the model O3 is more sensitive to the increased H2O than is the observed O3. The 20--30% decline in CH4 also implies a large increase in ClO which exceeds that expected from CFCs. Observations of ClO from the Microwave Limb Sounder (MLS) support this inference. ¿ 1998 American Geophysical Union