Several hydrazinium (1+) and dihydrazinium salts of aliphatic and aromatic carboxylic acids have been prepared by neutralisation of these acids with hydrazine hydrate in aqueous medium. They are hydrazinium hydrogendicarboxylates of malonic, succinic, glutaric and adipic acids, hydrazinium hydrogenphthalate monohydrate, hydrazinium carboxylates of benzoic, salicylic and p-hydroxybenzoic acids, and dihydrazinium dicarboxylates of succinic, phthalic, isophthalic and terephthalic acids. These salts have been characterised by analytical, IR spectral and thermal studies. IR spectra of all the salts show N-N stretching frequencies of the N2H5+ ion in the region 960-990 cm(-1). Most of the salts undergo endo thermic decomposition. Salts of benzoic acid and isophthalic acid show exothermic as well as endothermic decomposition whereas salt of terephthalic acid shows complete exothermic decomposition. All of them undergo complete decomposition to gaseous products.