Thiosulfinates are an emerging class of oxidized sulfur species that are frequently supposed to be involved in biochemical processes. Reaction of 12- and 10-membered ring pseudopeptidic thiosulfinates 1a (4,4,7,7-tetramethyl-1,3,4,7,8,10-hexahydro-5,6,1,10-benzodithiadiazacyc lododecine-2,9-dione 5-oxide) and 1b (3,3,6,6-tetramethyl-1,8-dihydro-4,5,1,8-benzodithiadiazecine-2,7(3H,6H) -dione 4-oxide) with a Ni(II) salt leads after ring cleavage under alkaline conditions to the isolation of diamidato/thiolato/sulfinato complexes. These two thiolato/sulfinato complexes of nickel, which can also be prepared by dioxygen oxidation of the parent diamidato/dithiolato complexes, were characterized by X-ray crystallography. They show a square-planar geometry with a S-bonded sulfinato ligand. A similar reaction between 1b and a Zn(II) salt leads to a thiolato/sulfinato complex with an O-bonded sulfinate via the intermediate formation of a mixed thiolato/sulfinic ester. On the basis of H-1 NMR, IR, and mass analyses, the sulfinic ester in the intermediate is proposed to be O-bonded to the zinc center. Then, an in-depth study of the cleavage of these thiosulfinates with the oxyanions RO- and HO- was performed. This led, after trapping of the open species with CH3I, to the identification of three polyfunctionalized products containing a methyl thioether, with either an isothiazolidin-3-one S-oxide, a methyl sulfone, or a methyl sulfinic ester. All of these products arise from a selective nucleophilic attack at the sulfinyl sulfur, promoted either directly by RO- or HO- or by an internal peptidic nitrogen of the thiosulfinate after deprotonation with RO- or HO-.