The behaviour of magmatic volatiles dictates various aspects of a subaerial volcano's eruptive style, morphology, and potential impact on the atmosphere. Typically, when volatiles bleed from nonerupting magma bodies, they encounter superimposed hydrothermal envelopes. If their interaction with hydrothermal fluids is minimal, subaerial degassing may be important, and possibly manifested in high fluxes of gaseous SO2. By contrast, a ''wet'' volcano, in which there is substantial mixing of magmatic and hydrothermal fluids, scavenges reactive magmatic components: sulfur-bearing minerals precipitate in crater lakes, in pore spaces created by hydrothermal dissolution of host rock, and in fractures formed by caldera collapse. Measurements of subaerial SO2 fluxes (e.g., by Correlation Spectrometer, COSPEC) therefore say as much about hydrothermal vigour as they do about magma bodies. Major explosive eruptions necessarily entail reaming of conduits through volcanic edifices, and entrainment of hyrothermal sulfur. It is argued here that eruption clouds might thereby derive significant proportions of their SO2 budget from combustion of native sulfur and/or sulfides sequestered by hydrothermal systems during inter-eruptive periods of >102--103 yr. ¿ American Geophysical Union 1996