Ellipsometry was used to observe the in situ growth and electrochemical conversion redox processes of a polypyrrole film, which was electrochemically deposited to a thickness of ca. 100 nm on a platinum electrode in 0.2M KCI solution containing 50 mM pyrrole. To determine the spatial distribution of oxidized and reduced states during the reduction and oxidation of the film, the conversion process was simulated by a multilayer film model. Such simulations of the ellipsometric results show that the redox conversion of a polypyrrole film proceeds from the solution toward the electrode and is controlled by counterion movement. These results contrast with previous work on a thionine film where conversion proceeds from electrode to solution. The effective diffusion constant for charge transport based on ellipsometric simulation was estimated as 1.7 x 10-10cm2/s.