Spectral aerosol optical depth, &tgr;a, observed at Mauna Loa, Hawaii, for the past 11 years is analyzed for background variations and the effects of two major volcanic eruptions: El Chich¿n in 1982 and Mount Pinatubo in 1991. A previously known annual variation and near-background levels are present in the record. The data are of high accuracy, being primarily obtained from an automatic precision sunphotometer and reduced using the Langley-plot slope method. The &tgr;a values over Mauna Loa were greater immediately after the eruption of El Chich¿n than after Mount Pinatubo due to more direct transport from El Chich¿n. However, Pinatubo had a greater temporally integrated impact because of greater erupted sulfur mass. A mean solar irradiance decrease of 6.5 (¿2.5) W m-2 per 0.1 &tgr;a (500 nm) averaged over 24 hours is observed for both volcanic eruptions. Slight differences are suggested between the eruptions, but the differences are not statistically significant. Small differences between the two eruptions in the aerosol size distributions derived from &tgr;a observations are also indicated and are consistent with the suggested difference in total solar irradiance aerosol sensitivity. The near-background &tgr;a values compare well with in situ surface-based aerosol light-scattering measurements extrapolated through the upper troposphere. ¿ American Geophysical Union 1994