Infrared spectroscopic studies on the fully reduced form of polyaniline are discussed in this work. Ex situ FTIR spectra measured in NaReO4 + HReO4 electrolytes with different pH values are presented, and the absorption bands are assigned to different vibrational modes. Additionally, a distinct picture of the base-acid transition is evaluated using in situ FTIR-ATR measurements in electrolytes containing the supporting salt NaReO4 + HReO4. The results demonstrate that in strong acidic media leucoemeraldine nitrogen atoms can be partly protonated. The protonation process starts in the range between pH = 2.5 and 3 and the intercalation of anions from the electrolyte into the polymer structure is determined semi-quantitatively.