ROBERT A. GOLDSTEIN Electric Power Research Institute. Environmental Assessment Department. P.O. Box 10412. Palo Alto. CA 94303. U.S.A. CARL W. CHEN Systech Engineering. Inc .. 3744 Mt. Diablo Boulevard. Suite 101. Lafayette. CA 94549. U.S.A. and STEVEN A. GHERINI Tetra Tech. Inc .. 3746 Mt. Diablo Boulevard. Suite 300. Lafayette. CA 94549. U.S.A. (Received November I, 1984; revised May 14, 1985) Abstract. An integrated, interdisciplinary, intensive study of three forested watersheds in the Adirondack Park region of New York State was started in 1977 to quantify the relationship between the deposition of atmospheric acids and surface water acidity. A general mechanistic theory of lake-watershed acidification that takes into account the production and consumption of acidity by watershed processes, as well as atmospheric inputs of acidity, was developed. This theory is formulated as a mathematical simulation model. Model and data analyses establish the importance of using an integrated ecosystem perspective to assess the vulnerability of surface waters to acidification by acidic deposition. The acid-base status of surface waters is determined by the interaction of many factors: soil, hydrologic, vegetation, geologic, climatic, atmospheric. The absolute and relative contribution of any given factor can be highly variable, both geographically and temporally; hence, lake sensitivity to changes in the quality and quantity of atmospheric deposition is also highly variable.