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Acid deposition can have significant ecological effects on peatlands, especially in fens near the limit of their buffering capacity. This study addresses the impact of exogenous acid on the highly organic peat soils of a Southern Appalachian fen. Soil samples were collected and analyzed for bulk density, percentage of organic matter, cation exchange capacity, base saturation, mineralogy, cations, anions, and buffering capacity, as determined by soil titration. Water samples were collected monthly for 10 mo and analyzed for pH, cations, and anions. The surface soils of the fen were well buffered, requiring about 8.5 cmol of H+ kg-1 to change the pH one unit, an amount much higher than values reported for forest soils. The buffer capacity is concentrated in the top few centimeters of the peat, with the deeper soils being much more susceptible to change. Buffer capacity is significantly correlated to the percentage of organic matter and cation exchange capacity. Almost all buffering of acid additions up to 2.0 cmol H+ kg-1 is the result of the organic matter, with the mineral fraction contributing only at much higher levels of acid addition (10.0 cmol H+ kg-1). Increased acidification of soils by the addition of H2SO4 results in the accumulation of SO42-. Sulfate retention is negatively correlated with percentage of organic matter and can be reversed by the addition of a strong base (NaOH). Field data corroborate the laboratory findings and indicate a negative correlation between SO42- and Ca2+ in surface water. This relationship suggests that the fen soils have not yet reached critical levels of pH for SO42- adsorption.