This paper proposes a volcanic SO2 index (VSI), on the basis of a 15-year satellite sampling of volcanic eruptions. This index is scaled to be compatible with the commonly cited volcanic explosivity index (VEI) used to compare explosive volcanic eruptions. A range of SO2 for each VEI level is proposed so that estimates of SO2 emission can be made from the historic VEI database. Volcanic SO2 from large eruptions can be routinely measured by satellite instruments, and for the smaller eruptions, where the sensitivity of satellite remote sensing is not sufficient, ground-based measurements are becoming more available. At the present time, we do not think SO2 can be used to define an accurate climate-perturbation index, as we have both insufficient data on volcanic SO2 and understanding of the volcano-climate relationship.¿ 1997 American Geophysical Union