Structure development in polyaniline (PAn) films was followed during the electrochemical polymerization in 1 M HCl solution via a constant potential mode at + 0.76 +/- 0.03 V and pH = 0.2 by using several in situ and/or out of situ techniques including UV-visible spectrometry, chronoamperometry, cyclic voltammetry, Rayleigh scattering and atomic force- and optical microscopy. As the polymerization proceeded from the initial nucleation stage I (where the polymerization current density varies as I proportional to t(0)) to the intermediate stages IIa and IIb (I proportional to t(3) --> t(2)) and the final stage III (I proportional to t(4)), the gross morphology changed from grainy to fibrillar texture. The fibrils were aggregates of the grains with the size unchanged from that formed in the early stage. This difference between the grainy and fibrillar structures is due to the difference in the state of aggregation of the initially formed grains. In the later stages, however, the fibrils began to form branches, accelerating the rate of polymerization. The branched fibrils grew toward the vertical direction, keeping their horizontal structural size virtually unchanged.