We examined soils along a transect across a submerged upland tidal marsh on the Eastern Shore of the Chesapeake Bay in Maryland. The soil was classified as a Terric Sulfihemist and was underlain by silty mineral sediments. Total S in the soil was partitioned into organic S, pyrite S, and soluble SO42−components. In the organic horizons, the ratio of pyrite S to total S was variable, ranging from 10 to 60%. The high levels of SO42−and organic matter in the peat, as well as the presence of free S2−, suggest that Fe is limiting pyrite accumulation in these horizons. Pyrite was identified in thin section by its spherical, framboidal habit and was confirmed in thin section and in grain mounts of heavy minerals using an electron microprobe. Individual or small clusters of framboids appeared to be randomly distributed through the organic horizons with few zones or pockets of high concentration. This was consonant with the low input of reactive (easily reducible) Fe (III) adsorbed to mineral sediments added to this isolated marsh. In the underlying mineral substratum, organic matter was thought to be limiting pyrite accumulation by controlling available energy for reduction. In this zone pyrite was concentrated in root channels where there had been some organic substrate for microbial sulfate reduction. The factors controlling pyrite reduction. The factors controlling pyrite accumulation in these tidal marsh soils may differ dramatically over distances as short as a few centimeters.