The electrochemical quartz crystal nanobalance has been used to study the formation and redox behavior of polyindole in acid media. It was found that the redox transformations of polyindole are accompanied with the sorption and desorption of counterions. A comparison is made with the electropolymerization of 4-aminoindole and with the behavior of poly(4-aminoindole), where completely different mass change curves have been detected. Based on the results of the nanogravimetric measurements a redox mechanism describing also the pH dependence is suggested. (C) 2013 Elsevier Ltd. All rights reserved.