The April 4, 1982, eruption of El Chich¿n southern Mexico introduced several megations of gaseous sulfur into the stratosphere, mostly as SO2 or H2S. Available kinetic information indicates that H2S is converted to So2 much faster than SO2 is oxidized to sulfate by homogeneous gas phase reactions. The process by which SO2 is chemically converted to sulfate is not precisely known and various reaction mechanisms have been proposed. The rate-limiting step of SO2 oxidation is believed to be reaction with the OH radical to form the HOSO2 radical. Depending on the fate of HOSO2, odd hydrogen may or may not be regenerated in the overall oxidation sequence. A decrease of OH due to volcanic SO2 should be observable through the gaseous species that depend on OH, and in particular SO2 itself. One- and two-dimensional model results are presented that estimate the photochemical effects of the SO2 from the El Chichon volcano under various assumptions of HOSO2 to sulfate conversion. It is found that the chemical lifetime for the volcanic SO2 would be greater than 100 days for a large portion of the cloud if HOSO2 does not regenerate odd hydrogen during conversion to sulfate and if hetergeneous losses of SO2 are not competitive. However, observations of sulfate formation and SO2 imply a chemical lifetime of 30--40 days, which is consistent with HOSO2 conversion regenerating odd hydrogen. The implications of this finding for the problem in the polluted troposphere is briefly reviewed.