The complexes [(CH3)(4)N](2)[Hg(SC6H5)(4)] and [(C4H9)(4)N][Hg(SC6H5)(3)] demethylate (CH3O)(3)PO as revealed by H-1, P-31{H-1}, and Hg-199{H-1} NMR spectroscopy in DMSO-d(6) solution. The products of the [(CH3)4(N)](2)[Hg(SC6H5)(4)] reaction are CH3SC6H5, (CH3O)(2)PO2-, and [Hg(SC6H5)(3)](-), whereas [Hg(SC6H5)(3)](-) demethylates (CH3O)(3)PO to yield CH3SC6H5 and {Hg(SC6H5)(2)[(CH3O)(2)PO2]}(-). Kinetic and solution studies of [(CH3)(4)N](2)[Hg(SC6H5)(4)] reveal a rapid equilibrium between bound and free thiolate. The dissociated thiolate is the nucleophile active toward (CH3O)(3)PO. These results imply that the metal center of the inactive mercury derivative of the Escherichia coli Ada DNA alkylation repair protein may comprise a three-coordinate [Hg(S-cysteine)(3)](-) moiety and an unbound, protonated cysteine (HS-Cys69).