Vertical profiles of HCl/Cly measured by space shuttle (Atmospheric Trace Molecule Spectroscopy (ATMOS)) and balloon (MkIV) instruments from 1985 through 1994 show that the increased volcanic aerosol from Mount Pinatubo had little effect on chlorine partitioning above 21 km. Below 21 km, however, large changes in HCl/Cly were observed. Aircraft (Aircraft Laser Infrared Absorption Spectrometer), balloon (MkIV, Far Infrared Spectrometer 2), space shuttle (ATMOS), and satellite (Halogen Occultation Experiment) measurements below 21 km reveal an increase in HCl/Cly between early 1993 and late 1997 of 31¿9%. This growth rate is 2 to 3 times larger than model estimates of the effect of the change in aerosol surface area and volume over this time period. For 1993 the five independent direct measurements of HCl/Cly produce a mean value of 0.57¿0.05, whereas by 1997 the mean value was 0.75¿0.07, in excellent agreement with the pre-Pinatubo mean of 0.74¿0.11. Measurements made in 1992 at the peak of the post-Pinatubo midlatitude aerosol loading demonstrate large variability, which is qualitatively consistent with model predictions. This variability appears to be attributable to the temperature sensitivity of the Cly partitioning when aerosol abundances are high. ¿ 2000 American Geophysical Union