Some new hydrazinium salts of benzene tricarboxylic and tetracarboxylic acids have been prepared by neutralisation of these acids with hydrazine hydrate in aqueous medium and characterised by conductance measurement, IR spectral and thermal analyses. Hemimellitic acid (H(3)hml) forms monohydrazinium salt, trimellitic acid (H(3)tml) and trimesic acid (H(3)tms), mono and dihydrazinium salts, and pyromellitic acid (H(4)pml) all the four salts with hydrazine hydrate. Conductance study indicates their electrolytic nature. IR spectra of all the salts show N-N stretching frequencies of the N2H5+ in the region of 960-990 cm(-1). The hemimellitate salt undergoes endothermic dehydrazination at 154 degreesC, trimellitates and pyromellitates in the range of 191-271 degreesC, and trimesates in the range of 267-332 degreesC. Trimesates decompose to give CO2 around 337 degreesC. All the salts then undergo strong exothermic decomposition in the range of 517-595 degreesC via the formation of respective acid intermediates first, then arenes, yielding carbon residue. A comparison of the thermal behaviour of pure acids with that of their salts reveals the fact that the acids do not withstand high temperature like salts, They show sharp endotherms at their melting points and then they decompose exothermally before 400degreesC to give carbon residue. (C) 2004 Elsevier B.V. All rights reserved.