A new method of in situ piezoelectric Fourier transform infrared (FTIR) spectroelectrochemisty, i.e., the combination of in situ FTIR and electrochemical quartz crystal microbalance (EQCM), was developed to study the electropolymerization of aniline and aniline-co-o-aminophenol, to investigate the properties of the polymers in 0.2 M HClO4. The piezoelectric electrochemical studies showed that the copolymerization process was changed in the presence of o-aminophenol and the copolymer exhibited different electrochemical behaviors from polyaniline and poly-o-aminophenol. The effects of the molar ratio of o-aminophenol on the copolymerization speed and the scan rate or pH values on the electroactivity of the copolymer were also investigated. The results suggested that the copolymer formed in the case of F = 0.1 had good stability and electroactivity than polyaniline at different pH values. The results obtained by the way of in situ piezoelectric FTIR spectroelectrochemisty indicated that the copolymerization process and the properties of the copolymer were different from that of polyaniline. The polymerization mechanisms and the structure of the two polymers were also different from each other. The copolymer formed through head-to-tail coupling of the two monomers via -NH- groups was a new polymer rather than a mixture of polyaniline and poly-o-aminophenol. (c) 2006 Published by Elsevier Ltd.