The eruption of Miyakejima Volcano, 180 km south of Tokyo, Japan, since July 2000 has resulted in the emission of large amounts of sulfur dioxide. The volumes of sulfur dioxide emitted were vast, equivalent to half the anthropogenic emission from China on annual average. Short-time aerosol sampling, conducted at the Happo Ridge observatory in the central mountainous region of Japan, has revealed that the abundant volcanic sulfate resulted in the expulsion of nonvolcanic nitrate and chloride in aerosols into the gas phase. As the deposition velocities of gaseous nitrate and chloride are much higher than those in aerosols, their deposition should be accelerated. This additional deposition is referred to as indirect acidification in this study, corresponding to direct acidification by the deposition of volcanic sulfate. This indirect acidification is estimated quantitatively using a regional-scale Eulerian aerosol transport model and is found to be generally less pronounced than direct acidification, although the indirect effect was 2.1 greater than the direct effect over the North Pacific Ocean in winter. The minimum contribution of indirect acidification is found to be 7%, which is still not negligible. Indirect acidification may also occur in the case of general air pollution, accelerating the acid deposition of nitrate and chloride in sulfate-rich contaminated air masses. It therefore appears to be important in general to consider indirect acidification in the discussion of environmental acidification by sulfur oxides.