The electrochemical processes involved in the oxidation of polyaniline in sulphuric and p-toluenesulphonic acids were studied under steady potential and during a potentiostatic current transient using UV-Vis absorption spectroscopy. It was found that sulphate ions are reversibly inserted into the polymer Γlm on oxidation and removed during reduction, while tosylate ions are irreversibly accumulated slowly in the Γlm and protons are exchange instead. The redox peaks involve two independent transformation equilibria of three main species : reduced species, polaron and bipolaron of di β erent chain lengths. While the charge involved in the polaron transformation is less than 20% of the charge involved safely in the oxidation of polyaniline before the onset of degradation, a signiΓcant capacitive contribution 40Γ60% was detected before the start of further oxidation of the polaron to bipolarons. Nernstian analysis of the redox process was possible only for the reverse scan and with the absorption peak of the reduced form. The spectroscopic transients showed that the oxidation, which is much slower than the reduction, is essentially the same in both acids without a signiΓcant contribution of the di β usion of dopant species in contrast with the reduction process.