Concern and controversy over the effects of acidic deposition on low ionic strength surface wa ters has led to much discussion on the nature and extent of proton transformations within acid sensitive ecosystems. The source of base neutralizing capacity(BNC) within acid surface waters has been attributed to atmospheric deposition of H2SO4 (or SO2) or HNO3, as well as production of soluble organic acids from soils. Unfortunately many of these studies have failed adequately to characterize aluminum, which is often a very significant component of BNC in acidic waters. The authors have evaluated the nature of short-term changes in the BNC of an acidic clearwater lake. The authors' results suggest that much of the variation in hydrogen ion and aluminum BNC can be attributed to changes in nitrate concentration, rather than to variations in sulphate, chloride, or organic anion concentrations.