An electrospray ionization (ESI) mass spectral characterization of eight substituted tris(dithiocarbamato)iron(III) complexes (Fe(III)(dtc) 3 ) and of the diethyldithiocarbamate (et 2 dtc) -, complexes of Mn(III), Cu(II), Ni(II), and Co(III) is presented. Use is made of the inherent electrochemical potential of the ESI source to oxidize the neutral metal complexes and in-source collision-induced dissociation (CID) to characterize the resulting ions. The mass spectra obtained at low source offset voltages all show molecular ions of the oxidized complexes, in addition to ions due to the loss of a dithiocarbamate, (dtc) -, ligand. Three of the Fe(III)(dtc) 3 complexes also show ions due to fragmentation of the (dtc) -ligands. Ions corresponding to the thiuram disulfide formed following the oxidation of excess (dtc) -in solution are also seen. In-source CID studies of the Fe(III), Mn(III), Cu(II), Ni(II) and Co(III) (et 2 dtc) -complexes all show [M(I)(et 2 dtc) Y H] Y ions attributed to proton transfer from a protonated solvent ion to a M(I)(et 2 dtc) neutral formed upon following CID in the high-pressure region of the ion source. The results of the present in-source oxidation studies are compared with those obtained in previous ESI studies in which external oxidizing agents were added to the metal complex solutions prior to analysis.