The mechanism whereby acid deposition can cause acidification of surface waters via equilibrium processes in soil solution was investigated using chemical equilibrium models. These models show that for soils with low to moderately low exchangeable bases the soil solution pH is only slightly affected by CO2 partial pressures over the range likely to be found in soils but the alkalinity of the soil solution increases rapidly with increasing CO2 partial pressure. In contrast, solutions that are not in contact with the soil's cation exchange complex maintain alkalinity independently of CO2 partial pressure. If alkalinity is positive, the pH in such solutions rapidly increases in response to decreasing CO2 pressure.