One of the most important biogeochemical processes in estuary sediment is microbial sulfate reduction, producing iron sulfide (pyrite) - the significant form of sedimentary sulfur. The oxidation of iron sulfide causes acidification of the environment. To study these biogeochemical processes, estuary water salinity and sulfate concentrations were determined and sediments from the Klong Ngao mangrove estuary were sampled during the August 2000 wet season. In situ sediment pH and the sediment oxidation-reduction potential were measured. Organic matter concentrations and total sulfur concentrations of the air-dried sediments and the chemical properties of the sediment water were investigated. The estuary water salinity and sulfate concentrations at the head were remarkably lower than at the mouth of the estuary. The in situ pH of mangrove sediments was neutral and the sediment metabolism was anaerobic. The sedimentary organic matter concentrations were relatively high. The amount of sulfur deposited in the sediments at the head was higher than at the mouth of the estuary. Dissolved iron and soluble sulfate, as well as an acid solution, were clearly found in the sediment water at the head of the estuary, whereas the sediment water pH at the mouth remained neutral apparently in association with the presence of soluble bicarbonate ions, and accordingly, dissolved iron was not found in the solution.