The interstitial water and sediment chemistry of cores from two different environments in Florida Bay, mangrove swamp and submerged mud bank, have been analyzed. The interstitial water profiles show that sulfate reduction, along with the resulting ammonium, phosphate, and alkalinity production, is occurring in Florida Bay sediments. However, the sulfate concentration profile is not typical of other anoxic environments, in that the sulfate concentration increases below a depth of approximately 20 cm. The ammonium, phosphate, and alkalinity concentrations all decrease below this depth. It is suggested that these interstitial water profiles might be explained in terms of a balance between the mixing of the interstitial water with the overlying seawater and decreasing rates of organic matter decomposition with depth. The rates of organic matter decomposition have been measured in laboratory experiments, which indicate that sulfate reduction and ammonium generation are much faster (a factor of 3 to 9 times) between 0 and 10 cm than between 10 and 20 cm. Organic carbon, organic nitrogen, and amino acid profiles show decreases as large as a factor of 2 in the organic content of Florida Bay sediments in the top 60 cm of the sediment column.