The acid-base reactivity of MS2(dmpe)(2), where M = Mo (1) and W (2) and dmpe = Me2PCH2CH2PMe2, was examined. Compounds 1 and 2 arise via the one-pot reaction of (NH4)(2)MS4 and dmpe. Protonation of these species gives the stable salts [MS(SH)(dmpe)(2)]X. The pK(a)'s of the Mo and W compounds are estimated to be 16.5 and 15.5, respectively. Protonation causes the M=S distances to diverge from 2.24 angstrom to 2.06 and 2.57 angstrom, whereas the Mo-P distances do not change appreciably. H-1 and P-31 NMR studies for [1H]BAr4F reveal that the proton exchange is competitive with the NMR time scale; at low temperatures, individual signals for both the parent disulfide and its conjugate acid can be observed, Treatment of 1 with excess HOTf liberates H2S to afford [MoS(OTf)(dmpe)(2)]OTf, which forms an adduct with CD3CN and regenerates 1 upon treatment with SH-/Et3N solutions. Consistent with its ready protonation, complex 1 is methylated, and the use of excess MeOTf gives [MoS(OTf)(dmpe)(2)](+) and Me2S in a rare example of double alkylation at a sulfido ligand.