Copper(II), nickel(II) and cobalt(II) perchlorate complexes of 5,5-dimethylcyclohexane-1,2,3-trione-2-(p-nitrophenyl-hydrazone) (HL1), 5,5-dimethyl-cyclohexane-1,2,3-trione-2-(p-chlorophenylhydrazone) (HL2), 5,5-dimethylcyclohexane-1,2,3-trione-2-(o-chlorophenylhydrazone) (HL4), 5,5-dimethylcyclohexane-1,2,3-trione-2-(o-methylphenyl-hydrazone) (HL5) and 5,5-dimethylcyclohexane-1,2,3-trione-2-(m-methylphenylhydrazone) (HL6) have been prepared, and characterized using analytical, spectral and magnetic measurements. The data reveal that the reaction of Cu(ClO4)2 (1 mol) in EtOH, with all ligands, produces complexes of the type CuL(ClO4)(H2O).nH2O. Nickel(II) and cobalt(II) perchlorates react only with HL1 and HL2 to produce the complexes ML(ClO4)(H2O)3 (where M = NiII, L = L− and L2, M = CoII, L = L1) and Co(HL2)2-(ClO4)2.2H2O. The spectral data show that the ligands behave as monobasic bidentate in their azo forms, except HL2 which reacts with cobalt(II) as a neutral bidentate ligand in its hydrazone form.