Abstract Metal o- or p-hydroxybenzoate hydrazinate complexes of the types: Mn( o-C 6H 4 (OH)COO) 2 N 2H 42H 2O; ML 2N 2H 4, where M is Cu(II), L is o-hydroxybenzoate(salicylate) and M is Zn(II), L is o- or p-hydroxybenzoate; ML 22N 2H 4, where M is Co(II) and Ni (II) and L is o- or p-hydroxybenzoate, and bishydrazinium metal salicylates (N 2H 5) 2M( o-C 6H 4 (OH)COO) 4, where M is Co(II) and Ni(II), have been prepared and characterised by magnetic, spectral, thermal and XRD studies. IR spectral data reveal the monodentate coordination of hydroxybenzoates through COO −, coordination of N 2H + 5, bidentate bridging of N 2H 4 and coordination of H 2O in the Mn complex. The magnetic and electronic spectral data indicate octahedral coordination around Mn, Co and Ni and square-planar coordination around Cu ions. The simultaneous TG-DTA data show that Ni and Cu salicylate hydrazinate complexes undergo complete exothermic decomposition, whereas other complexes decompose both by endothermic and exothermic modes to produce metal oxide as the residual product. Most of the complexes form metal hydroxybenzoates as intermediates. Co and Ni intermediates melt before decomposition. The XRD studies show isomorphism among ML 22N 2H 4, ZnL 2N 2H 4 and (N 2H 5) 2M( o-C 6H 4(OH)COO) 4 complexes but not between the series.