The conversion rate of polyaniline films from various doping levels into the fully conducting state would be expected intuitively to increase with the doping levels because higher conductivity due to the doping enhances charge transport in the film. However, it showed a maximum when the doping level was varied electrochemically. The conversion rate was evaluated from the time-variation of the absorption of an He-Ne laser beam in response to a potential step, in order to avoid capacitive effects. The absorption increased linearly with time immediately after the electrochemical switching, The linearity agrees with the theory of the propagation of the conducting zone. The initial rates varied with the electrode potential before the step. The rate vs. potential curve passed through a maximum at a potential between the fully insulating domain and the fully conducting domain. By taking into account that the conducting species grows at the boundary between the resistive zone and the conducting zone with the electric connection to the electrode, the appearance of the maximum was explained by percolation theory in which the ar ea of the conducting clusters reaches a maximum near the percolation threshold. Dynamic observation using a microscope with a magnified view of the film showed that the boundary on the cross-section of the film normal to the electrode surface had a complicated shape. It was reproduced by Monte Carlo simulation of the conversion in a two-dimensional square lattice model,