The chapter discusses the major chemical processes by which acidic deposition interacts with soils. The focus is on forest soils, as the effects of acidic deposition on soils used for production of food and fiber are generally small compared to effects of agricultural practices such as nitrogen fertilizer applications and liming. Buffering mechanisms considered include aluminum buffering, silicate mineral buffering, cation exchange, organic buffering, and the effect of anion immobilization processes such as nitrate uptake and sulfate adsorption. The effects of acidic inputs on capacity factors such as exchange acidity, exchangeable base content, and sulfate adsorption capacity are considered, as are related natural processes such as acidification due to accumulation of bases in biomass. Particular attention is paid to intensity effects, such as the effect of increased concentration of anions associated with strong acids on the chemical composition of the soil solution, as they are likely to be highly nonlinear with respect to the capacity factors. These include pH, aluminum mobilization, and loss of alkalinity in the soil solution, which in turn may result in acidification of drainage waters.