We report a structural characterization using X-ray absorption spectroscopy of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) reductase reduced with trimethylarsine and show that this is structurally analogous to the physiologically relevant dimethyl sulfide reduced DMSO reductase. Our data unambiguously indicate that these species should be regarded as formal Mo-IV species and indicate a classical coordination complex of trimethylarsine oxide, with no special structural distortions. The similarity of the trimethylarsine and dimethyl sulfide complexes suggests, in turn, that the dimethyl sulfide reduced enzyme possesses a classical coordination of DMSO with no special elongation of the S-O bond, as previously suggested.