The effects of atmospheric deposition on acid-sensitive watersheds have become increasingly apparent. Lake/watershed systems that cannot completely neutralize strong acid inputs are characterized by low pH values and elevated concentrations of trace metals. Populations of fish and other aquatic biota are endangered by this phenomenon. One approach used to mitigate the effects of surface water acidification is direct application of calcium carbonate (CaCO3). Through the Lake Acidification Mitigation Project (LAMP), we investigated the chemical response of acidic lakes to base treatment. Immediately following base application, there was a marked increase in pH, acid neutralizing capacity (ANC), calcium and dissolved inorganic carbon (DIC) associated with the dissolution of calcium carbonate in the treated lakes. The large increase in pH was attributed to the low dissolved inorganic carbon concentrations in the water column prior to liming and limited pH buffering capacity. During the four week period following base application the intrusion of atmospheric carbon dioxide facilitated additional dissolution of the remaining suspended calcium carbonate. This dissolution was accompanied by a gradual decrease in pH (to below 8) and increases in acid neutralizing capacity, dissolved calcium and dissolved inorganic carbon concentrations within the lakes. Concentrations of trace metals in the upper waters decreased about one order of magnitude due to reduced solubility at circumneutral pH values. The rate of reacidification was directly related to the hydrologic input to the lake. Elevated discharge during the fall coinciding with a completely mixed water column greatly facilitated reacidification. Although discharge was very high during spring snowmelt, ice cover and inverse thermal stratification restricted intrusion of acidic meltwater to the ice-water interface. The rapid rate of reacidification was largely attributed to the shallow depth and short hydraulic retention of these lakes.