The polar-orbiting Special Sensor Microwave Imager (SSM/I) sensor was used to collect passive microwave radiation (19-85 GHz) for the August 19, 1992 Crater Peak/Spurr volcanic cloud. This eruption was also imaged by a ground based C-band radar system at Kenai, Alaska, 80 km away, and by the thermal infrared channels of the polar-orbiting Advanced Very High Resolution Radiometer (AVHRR). The SSM/I sensor detects scattering by volcanic ash particles with a radius larger than about 0.1 mm. The size of ash particles in a volcanic cloud can be estimated by the comparison of the scattering observed in the various microwave channels. The mass of particles in the volcanic cloud can be estimated by adapting empirical methods used for rainfall rate estimates con sidering the differing dielectric constants of volcanic ash and raindrops. For the August 19, 1992 Crater Peak/Spurr eruption the SSM/I based estimate of ash fallout mass (3 x 1010 kg) was about 85% of the mass measured by field geologists of the ash fallout blanket. Like weather radar systems, the SSM/I offers the ability to sense volcanic clouds during and immediately following (within 30 minutes) actual eruptions. Because most volcanoes are out of range of weather radar systems, the SSM/I may be an important tool for sensing the magnitude, initial trajectory and potential fallout mass of eruptions, and could therefore play a role in mitigating volcanic cloud hazards for aircraft.