Acid neutralizing capacities (ANC) of sediments from McCloud Lake, Florida and seven other lakes in Wisconsin and Florida were as high as 10 meq 100 g-1 over the pH range 4.5 to 5.5 in well-mixed batch experiments. Exchange of Ca2+ and Mg2+ accounted for over 50% of the neutralizing capacity; Al solubilization and sulfate adsorption were unimportant in neutralizing H+ additions. ANC was correlated with sediment volatile solids content. Sulfate reduction occurred in microcosms that simulated lake-water interactions and subsurface seepage;in situ pore water profiles and a whole-lake mass balance confirm the occurrence of this process in McCloud Lake. Sediment neutralization is important for lakes that receive most of their water from precipitation and thus are particularly susceptible to acidification.