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New mixed-ligand complexes of the general formula, [M(L)(sac)] (M = Ni2+, Cu2+; L− = anionic forms of the salicylaldehyde Schiff base of N-methyl-S-methyldithiocarbazate (Hsalsme), the salicylaldehyde 2-N-methyl-3-thiosemicarbazone (Hsaltsc), the methylpyruvate Schiff base of S-methyldithiocarbazate (Hmpsme) and the quinoline-2-methoxycarboxaldehyde Schiff base of S-methyldithiocarbazate (quinolsme); sac− = the saccharinate anion) have been synthesized and characterized by IR, electronic and magnetic susceptibility measurements. Magnetic and spectroscopic data support a distorted square-planar structure for the [M(ONS/NNS)(sac)] complexes (ONS/NNS = salsme, saltsc, mpsme or quinolsme). The structures of Hsalsme, [Cu(salsme)sac] and [Cu(quinolsme)sac] have been determined by X-ray diffraction. The complex, [Cu(salsme)sac] has a distorted square-planar structure with the Schiff base acting as a uninegatively charged ONS tridentate chelating agent coordinating the copper(II) ion via the phenolic oxygen, the azomethine nitrogen and the thione sulfur atoms, the fourth coordination position being occupied by an N-bonded saccharinate anion. The reaction of [Cu(sac)4(H2O)2]·2H2O with the 2-quinolinecarboxaldehyde Schiff base of S-methyldithiocarbazate (Hqsme) in boiling methanol does not lead to the formation of the expected mixed-ligand complex, [Cu(qsme)(sac)], but an unusual substitution of the hydrogen atom attached to the azomethine carbon by a methoxy group occurs with the concomitant formation of the complex [Cu(quinolsme)(sac)] of a new ligand (quinolsme = anionic form of the quinolone-2-methoxycarboxaldehyde Schiff base of S-methyldithiocarbazate). An X-ray crystal structure determination reveals that the Schiff base Hquinolsme is coordinated to the copper(II) ion in its deprotonated thiolate form as an NNS tridentate chelating agent and the fourth coordination position of the square-planar copper(II) centre is occupied by an N-bonded saccharinate anion.