The effects of acid deposition on surface waters in eastern United States watersheds having similar size, physiography, climate and land use are related to the composition of the underlying bedrock. Watersheds developed on greenstone, calcareous shale, sandstone, granite, and schist differ in their ability to neutralize acid deposition. Surface waters in watersheds developed on greenstone and calcareous shale are not discernably affected by acidification. Wastersheds developed on sand-stone have little capacity to neutralize acid rain; consequently, stream acidity is similar to that of precipitation. Watersheds developed on granite and schist are intermediate in their capacity to neutralize acid deposition. Bedrock composition appears to be the major property controlling surface-water chemistry in these systems; hydrologic flow paths and the nature of surficial materials and vegetation also influence chemical responses to acid deposition in watersheds.