The potential utility of55Fe as a tracer for Fe(III) reduction in aquatic sediments was evaluated using freshwater sediments from the Potomac River, Md, in which Fe(III) reduction was the predominant terminal electron accepting process. In laboratory incubations of sediments amended with poorly crystalline55Fe(III) oxide, the accumulation of55Fe in Fe(II) over time paralleled Fe(II) production. However, within 1 h of addition of55Fe(III) oxide, significant quantities of55Fe were recovered in extractable (1 M sodium acetate, pH 5) and soluble Fe(II) pools. There was proportionately more55Fe(II) activity in sediments with higher Fe(II) concentrations. These results suggest that there was rapid isotopic exchange between55Fe in poorly crystalline Fe(III) oxide and the Fe(II) pools. Because of this exchange,55Fe could not be used to trace Fe(III) reduction in sediments in the manner that35SO42−is used to trace sulfate reduction. Although accumulation of55Fe in Fe(II) pools paralleled unla‐beled Fe(II) production, given the relative ease with which Fe(II) pools in sediments can be measured there is no apparent advantage to using55Fe to follow Fe(III) reduction.