The flavoprotein Tn501 mercuric reductase (MerA) catalyzes the reduction of Hg(II) to Hg(0) through the intermediacy of the tightly bound two-electron-reduced cofactor FADH-. To gain insight into the MerA mechanism, the interaction of the holoenzyme or free FADH- with various metal ions was investigated. The free two-electron-reduced FAD cofactor, FADH-, readily reduces a variety of metal ions, provided they have suitably high redox potentials. For Hg(II) with various ligands, the rate of reduction is inversely proportional to the stability of the Hg(II)-ligand complex. These results are consistent with the free cofactor reducing metal ions by an outer-sphere electron transfer mechanism. In contrast, MerA can tightly bind several redox labile metal ions, but only Hg(II) is reduced. The inability of MerA to reduce these bound metal ions may suggest that MerA differs from free FADH- and utilizes an inner-sphere electron transfer mechanism in Hg(II) reduction.