A polyaniline (PAn) film which has been electrochemically inactivated by continuous potential cycling in LiClO4/propylene carbonate can be reactivated by 2,5-dimercapto-1,3,4-thiadiazole (DMcT) or methanesulfonic acid. PAn reactivated by DMcT is in its reduced form, while that reactivated by methanesulfonic acid is in its oxidized form. Thus, the reversible inactivation is ascribed to a deprotonation of oxidized PAn. Additionally, DMcT can reduce the oxidized PAn. Similar results were obtained for. poly(N-methylaniline).