The explosive June 1991 eruptions of Mount Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption. The SO2 cloud observed by the TOMS encircled the Earth in about 22 days (~21 m/s); however, during the first three days the leading edge of the SO2 cloud moved with a speed that averaged ~35 m/s. Compared to the 1982 El Chich¿n eruptions, Pinatubo outgassed nearly three times the amount of SO2 during its explosive phases. The main cloud straddled the equator within the first two weeks of eruption, whereas the El Chich¿n cloud remained primarily in the northern hemisphere. Our measurements indicate that Mount Pinatubo has produced a much larger and perhaps longer-lasting SO2 cloud; thus, climatic responses to the Pinatubo eruption may exceed those of El Chich¿n. ¿American Geophysical Union 1992