The redox mechanism of polyaniline (PANI) film in acid solution was studied by in situ rapid-scan time-resolved infrared spectroelectrochemistry (RS-TR-FTIRS) method. In the potential rang between -0.29 and 0.91 V. there are four pairs of redox peaks. These four pairs of redox peaks were minutely investigated by cyclic voltammetry (CV) and in-situ infrared spectroelectrochemistry. The results illustrated that the first pair of redox peaks are according to oxidation of the secondary amines in the middle of molecular PANI chain, the second pair of redox peaks are according to oxidation of terminal primary amines, the third pair of redox peaks are according to oxidation of terminal hydroxyl groups, the forth pair of redox peaks are according to oxidation of the polaronic PANI to the bipolaronic PANI. The bipolaronic PANI completely turns to quinoid structure with highly conjugated by deprotonation when the electrode potential was higher than 0.71 V. Meanwhile, the hydrogen bonding between the oxidation products of PANI and water molecules disappeared. (C) 2015 Elsevier B.V. All rights reserved.