New hydrazinium hydrogen-4,5-imidazoledicarboxylate (N(2)H(5)Himdc), its monohydrate, N(2)H(5)Himdc.H2O, (H-2 imdc = 4,5-imidazoledicarboxylic acid) and the metal complexes of the type M(imdc)0.5N(2)H(4).H2O, where M = Mn,Co or Cd and M(imdc)N2H4.H2O, where M = Ni or Zn have been prepared. The compounds have been characterised by analytical, electronic and IR spectral and thermal studies. Electronic spectral data suggest that the Co and Ni complexes are of spin-free (high spin) type with octahedral geometry. The IR absorption bands of N-N stretching in the range 970-960 cm(-1) unambiguously prove the bidentate bridging nature of the N2H4 ligands. IR spectra also confirm the unidentate coordination behaviour of carboxylate (v(asy) = 1570 cm(-1) and v(sym) = 1390 cm(-1)) groups of the imidazoledicarboxylate dianion. Simultaneous TG and DTA of the free acid show a single-step complete endothermic (281 degreesC) decomposition whereas its anhydrous and hydrated hydrazinium salts show endotherm followed by exotherm. decomposition. All the metal complexes undergo endothermic decomposition around 100 degreesC with the loss of hydrazine and water, via the metal 4,5-imidazoledicarboxylate which further decomposes exothermically in the range 110-560 degreesC to give the respective metal oxide as the end product.