The redox dynamic of electrochemically prepared thin films of polypyrrole is studied. A semi-empirical formula to explain the redox behavior is formulated and it seems applicable to the reduction dynamic. The redox dynamic is dominated by a diffusion process, depending on the relation between the film thickness and the rise time (t(1/2)) of the reduction process. The diffusion coefficient, D, was measured to be 2 x 10(-9) cm(2)/sec for perchloride (ClO4-) anion dopant and 1.1 x 10(-9) cm(2)/sec for para-toluenesulfonate (PTS-) anion dopant. The polypyrrole films exhibited a color change during the oxidation and reduction processes. The electrooptical properties of these films are studied.