Abstract: Long-term changes in sulfate concentrations and soil acidification in two adjacent forest soils (Andosols and Umbrisols) were investigated using soil samples archived from 1967 to 2009. The amounts of phosphate-extracted sulfate in Andosols (≤51.0 mmol kg−1) were generally higher than that in Umbrisols (≤13.7 mmol kg−1). Both soils contained extremely high amounts of sulfate compared with the soils of polluted regions of Europe and North America in the 1980s and 1990s. A strong positive correlation was obtained between iron oxide minerals (acid oxalate–extractable Fe minus pyrophosphate-extractable Fe [Feo - Fep]) and phosphate-extracted sulfate, whereas soil organic matter was suggested to inhibit sulfate adsorption especially by allophane (acid oxalate–extractable Al minus pyrophosphate-extractable Al [Alo - Alp]). The amounts of phosphate-extracted sulfate in Umbrisols have gradually decreased by about one third between 1967 and 2009, resulting from the decline of sulfur emissions. However, those in Andosols have remained at high levels, indicating the very slow or irreversible equilibration of adsorbed sulfate in these soils. In addition, surface soils of Andosols from 1994 and 2009 showed soil acidification indicated by the decrease in soil pH, the dissolution of aluminum, and the formation of the precipitates, such as aluminum hydroxysulfate and basic iron sulfate. It is considered that irreversible adsorption of sulfate is an important factor contributing to the decrease in acid-neutralizing capacity. Although acidification in Andosols is slow to appear because of their high acid-neutralizing and sulfate adsorption capacities, acidification persists for a long time once the critical loads of acid deposition have occurred.