Measurements from the satellite-based special sensor microwave imager (SSM/I) were used to collect passive microwave radiation (19--85 GHz) for the August 19, 1992 (UT date), 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 (A VHRR). The SSM/I sensor detects scattering of Earth-emitted radiation by millimeter size volcanic ash particles. The size of ash particles in a volcanic ash cloud can be estimated by comparing the scattering at different microwave frequencies. The mass of particles in the volcanic ash cloud can be estimated by using a theoretical method based on Mie theory or by adapting the empirical methods used for estimating rainfall rates and accounting for the different 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 (1.3¿109-3¿1010 kg) was 4%--85% of the mass fallout measured in the field. Like weather radar systems, the SSM/I offers the ability to sense young volcanic ash clouds during and immediately following (within 30 min) actual eruptions. Because most volcanoes are out of range of weather radar systems, the SSM/I may be an important tool for determining the magnitude, initial trajectory, and potential fallout mass of eruptions. The SSM/I may therefore play a role in mitigating volcanic cloud hazards for aircraft, determining masses where ground sampling is not possible, and in issuing fallout warnings for communities downwind of volcanic eruptions. ¿ American Geophysical Union 1996