The ion exchange of polyaniline thin films during the leucoemeraldine-emeraldine transition was investigated for different concentrations of HClO4 using an electrochemical quartz crystal microbalance (EQCM). At pH 2 the mass increase on oxidation indicates that anions are inserted in order to compensate for the positive charge in the oxidized polymer. At higher acid concentrations (> 2 M), a clear mass decrease is observed at the beginning of the first oxidation step. This constitutes direct evidence that protons are expelled at the beginning of the first oxidation step. For 4 M HClO4 the molar mass equivalent of the species exchanged is close to -20 g mol-1, which would correspond to the mass of an expelled hydronium ion. The two distinct slopes observed in the frequency-charge plots can be used for an improved quantitative analysis of the relative contributions of ClO4- and protons to the overall ion exchange process in the first oxidation step of polyaniline.