Camphorsulfonic acid (CSA) and m-cresol are known to be one of the most effective pairs of dopant and solvent for polyaniline (PANI) doping, respectively. Although the concept of secondary dopant that modifies the conformation of PANI has been a major description for the role of m-cresol, the acidity of m-cresol also has been suggested as one of the crucial factors enhancing electron mobility of PANI. A m-cresol molecule is small enough to diffuse into PANI cluster, and it has ability to exchange a proton with a camphorsulfonate ion and PANI, while CSA is relatively big to stay in PANI crystalline region. Experiments and semiempirical quantum mechanics calculations were used in this study to investigate the role of acidic m-cresol in PANI. When 3-methylphenolate ions stay in PANI cluster, energy band gap is predicted to decrease for polaronic and bipolaronic structures, while undissociated m-cresol does not work for doping. (C) 2009 Elsevier Ltd. All rights reserved.