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Abstract: The reaction of phthalaldehyde (1,2-benzenedialdehyde) with S-methyldithiocarbazate does not lead to the formation of the expected open-chain linear Schiff base, phthalaldehyde bis(S-methyldithiocarbazonate) but a cyclic diazine dithiocarbazate is obtained which has been characterized by spectroscopic and X-ray diffraction. However, if the reaction of phthalaldehyde with S-methyldithiocarbazate is carried out in the presence of nickel(II) and zinc(II) acetate salts, stable metal complexes of empirical formula, [M(phthalsme)] (M=Ni, Zn; phthalsme=doubly deprotonated form of the Schiff base, phthalaldehyde bis(S-methyldithiocarbazonate) are obtained. Likewise, the reaction of 4N-methyl-3-thiosemicarbazide with phthaladehyde in the presence of nickel(II) acetate also yields a crystalline nickel(II) complex, [Ni(phthaltsc)] (phthaltsc=doubly deprotonated form of the teradentate phthaladehyde bis(4N-methyl-3-thiosemicarbazone) ligand. The complexes, [Ni(phthalsme)], [Ni(phthaltsc)] and [Zn(phthalsme)(DMF)] have been characterized by a variety of physico-chemical techniques and their structures have been determined by X-ray diffraction. In these complexes, the ligands are coordinated to the metal ion as dinegatively charged tetradentate chelating agents via the two thiolate sulfur atoms and the azomethine nitrogen atoms. The nickel complexes, [Ni(phthalsme)] and [Ni(phthaltsc)] exhibit a distorted square–planar geometry whereas the zinc complex, [Zn(phthalsme)(DMF)] is five-coordinate and square-pyramidal with the dianionic Schiff base occupying the basal plane of the square-pyramid and the oxygen atom of dimethylformamide in the apical coordination site. [Copyright &y& Elsevier]