Department of Chemistry, Panskura Banamali College, Midnapore-721 152, West Bengal, India Department of Chemistry, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata-700 009, India Manuscript received 2 April 2006. revised 27 November 2006. accepted 27 November 2006 The title ligand, 5-methy1-3-formylpyrazole 4N-benzylthiosemicarbazone (abbreviated as HMPzNB) has been synthesised and physicochemically characterised by elemental analysis, IR, PMR and mass spectrum. The ligational behaviour of the pyrazole-derived thiosemicarbazone, HMPzNB, has been examined by isolation and characterisation of its complexes with CoIII , NiII and CuII with different counterions. Elemental analyses, magnetic and spectral features indicate [Co(MPzNB)2]X (X = CI-, Br-, NO-3), CIO-4 or BF-4) as the diamagnetic octahedral species with the ligand in the deprotonated thiol form, while [Ni(HMPzNB)2]X2 (X = CI-, Br-, NO-3 or CIO-4) is proposed as octahedral complex with the ligand in the neutral "thione" form. However, in the neutral condition (pH ~ 6) nickel(ll) salt affords octahedral bis- ickel(ll) complex, Ni(MPzNB)2, while copper(ll) salt gives [Cu(MPzNB)]X (X = CI-, Br-, NO-3 or CIO-4), with the deprotonated ligand. 5-Methyl-3-formylpyrazole 4N-benzylthiosemicarbazone possesses in its structure, the tertiary nitrogen atom (2N) of the pyrazole ring, the azomethine nitrogen and the sulphur atom as potential binding sites; thus the title ligand is expected to behave as a potential NNS tridentate during complex formation with a metal ion