Sulfate adsorption by forest soils modifies the impact of pollutant deposition on cation leaching processes. We examined relationships among SO4-2 adsorption, soil properties, and seasonal variation in soil solution chemistry at 13 sites representing deciduous forest ecosystems common in the Great Lakes region. Objectives of the study were to test the validity of previously proposed SO4-2 adsorption indices, to examine within- and among-site variability in SO4-2 adsorption potential, and to investigate the effects of seasonal changes in soil solution chemistry on SO4-2 retention. Mineral soils were sampled by horizon at all sites, and soil solutions were sampled at lower E and lower B horizon boundaries at 10 sites. Propsed indices overpredicted sulfate adsorption in certain SO4-2 releasing subsurface horizons, seriously limiting the applicability of the published regression equations. We developed improved regression equations using the sum of initial extractable SO4-2 and additional SO4-2 adsorbed under laboratory conditions as the dependent variable. Sulfate retention indices predicted by the improved equations were independent of existing levels of extractable SO4-2 and past history of atmospheric SO4-2 deposition. Examination of within- and among-site variability in SO4-2 adsorption potentials suggested that soils need to be grouped tightly on a taxonomic basis for modelling purposes. Seasonal variation in soil solution SO4-2 concentrations and fluxes was consistent with an annual cycle of SO4-2 retention and release. Although seasonal patterns in SO4-2 concentrations and fluxes appeared to be controlled by hydrologic and S-cycling processes, the magnitude of SO4-2 fluxes was primarily related to atmospheric SO4-2 deposition rates.