New dihydrazinium divalent transition metal trimellitate hydrates of empirical formula (N2H5)2M(Html)2·nH2O, where n = 1 for M = Co or Ni, and n = 2 for M = Mn, Zn, or Cd (H3tml = trimellitic acid), and monohydrazinium cadmium trimellitate, [(N2H5)Cd(Html)1.5·2H2O] have been prepared and characterized by physico-chemical methods. Electronic spectroscopic, and magnetic moment data suggest that Co and Ni complexes adopt an octahedral geometry. The IR spectra confirm the presence of monodentate carboxylate anion (Δν = νasy(COO−) − νsym(COO−) > 190 cm−1) and coordinated N2H5+ ion (νN–N 1015 − 990 cm−1) in all the complexes. All the complexes undergo endothermic decomposition eliminating CO2 in the temperature region 200–250 °C, followed by exothermic decomposition (in the range of 500–570 °C) of organic moiety to give the respective metal carbonate as the end products except nickel and cobalt complexes, which leave respective metal oxides. X-ray powder diffraction patterns reveal that Ni and Co complexes are isomorphous as are those of, Zn(II) and Cd(II) of the type, (N2H5)2M(Html)2·2H2O.