In situ, spectroelectrochemical studies on the copolymerization of aniline (ANI) with diphenylamine-3-sulphonic acid (DPASA) were carried out in 0.5 hi II,SO, for different feed ratios of ANI and DPASA using ITO as the working electrode. Studies on homopolymerization of DPASA and ANI were also done independently in 0.5 M sulphuric acid. UV-Vis absorption spectra were recorded concurrently during the copolymerization and homopolymerizations. The intermediate species for the homopolymerization of DPASA and copolymerization with ANI have been identified by spectroelectrochemical studies. The spectroelectrochemical results revealed the formation of an intermediate at the initial stages of copolymerization through the reaction of the DPASA cation radical and the aniline cation radical to result in a head-to-tail dimer. This N-phenyl-para-phenylene diamine (PPD) type of intermediate was assigned to have a peak at 550 nm in the UV-Vis spectra. The assignment of a peak at 550 nm to the intermediate was further confirmed by deducing derivative cyclic voltabsorptogram (DCVA) at 550 nm. The results from constant potential electropolymerization also supported the formation of an intermediate that absorbs at 550 nm (C) 2001 Elsevier Science Ltd. All rights reserved.