The conducting zone of polythiophene films grew under the propagation-control in acetonitrile solution at application of the positive potential to the reduced film whereas it grew under the diffusion-control when it was transferred into the propylene carbonate solution. The electrochemically polymerized film was peeled off from the electrode, and a current feeder was connected to an end of the film. The distribution of the conducting species was detected with a diode array detector through the 850 nm beam irradiated to the film. When the fully reduced film was oxidized potentiostatically in the 0.1 M LiClO4 + acetonitrile solution, the conducting zone developed linearly with the electrolysis time at almost a constant speed. This is in accord with the theory of the propagation of the conducting zone. When the reduced film was transferred into the 0.1 M LiClO4 + propylene carbonate solution, the conducting zone grew exhibiting a vague boundary. Quantitative analysis said that the growth obeyed at first the propagation theory and then obeyed the square root of the time, represented by diffusion of dopant ions. The diffusion coefficient was 3x10(-10) cm(2) s(-1). A new growth model was proposed, in which the film/solution interface was oxidized at first by the propagation mechanism and then the conducting zone was dispersed into the film center by diffusion.