A detailed study of the origin and meaning of the anodic pre-peak, A(0), was carried out for polyaniline (PANI) films, which were electrochemically grown using the potential sweep method to two different upper potential limits (Type I and Type IT films) on Au, Pt and GC electrodes. The observed presence of a matching cathodic peak, C-0, for slowly grown Type I films, and the absence of the A(0) pre-peak for rapidly formed Type II films, suggest that the pre-peak arises from the oxidation of particular PANT sites within these films, likely located near the underlying substrate. Time of cycling over the A(0)/C-0 potential range (0 and 0.3 V) and/or holding of the potential in the reduced film state causes the A(0) peak potential to shift. Simultaneous cyclic voltammetry, quartz crystal microbalance and ellipsometry measurements indicate that the positive movement of the A(0) potential is related to film swelling. Long times of cycling between 0 and 0.6 V appear to cause the Type I film to anneal, resulting in a more compact PANI film, and a negative shift of the pre-peak. The pre-peak, therefore, appears to be a barometer of film water and/or solution anion content.