Department of Chemistry, University College, Thiruvananthapuram-695 034, Kerala, India E-mail : drmlhnair@gmail.com Department of Chemistry, All Saints' College, Thiruvananthapuram-695 007, Kerala, India Manuscript received 21 January 2010, revised 29 September 2010, accepted 05 October 2010 A few mononuclear complexes of oxomolybdenum(v) and dioxomolybdenum(VI) with a Schiff base derived from 4-aminoantipyrine and o-vanillin have been synthesized and characterized by elemental, spectral (IR, electronic, ESR and NMR) analysis, conductance measurements and cyclic voltammetric studies. The complexes have general formulae [MoO(AAPOV)CI2X] and [MoO2(AAPOV)CIX] where AAPOV = 2,3-dimethyl-1-phenyl-4-(2-hydroxy-3-methoxy benzylidene amino)pyrazol-5-one and X = CI, NCS, NO3 or CIO4 The ESR spectrum of the complex [Mo0(AAPOV)CI3] in the solid state exhibits a single line pattern. The ESR parameters were calculated and the gav • value (1.9795) shows that the complex is monomeric with Mo in the +5 oxidation state. The X-ray powder diffraction patterns of the complex [MoO(AAPOV)CI3] have been examined and found to be orthorhombic with unit cell dimensions a = 16.8088 Å, b = 11.7467 Å and c = 7.4816 Å. The thermal decomposition behaviour of the complex [MoO(AAPOV)Cl3] was studied using TG and DTG techniques. The complex is stable up to 270oc and then undergoes decomposition in three stages. The kinetic parameters were calculated using the Coats-Redfern equation. The cyclic voltammetric profile of the complex [MoO(AAPOV)(NO3)CI2] shows a quasi-reversible peak attributable to MoV/MoV1 redox system. The energy minimized computer model of the ligand and one of its complexes were also constructed.