The effect of molecular aniline on the potential of electrodes coated with a thin layer of polyaniline and immersed in aqueous neutral buffer solution has been studied. A theoretical equation describing the dependence of the potential on the concentration of aniline has been derived and shown to be in conformity with the experimental results. The theory is based on the formation of charge-transfer complexes between the aniline molecule and the oxidized form of polyaniline. (C) 2003 Elsevier Science B.V. All rights reserved.