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We report in situ electrochemical atomic force microscopic (EC-AFM) data and spectroelectrochemical results on doping−undoping of the films of regioregular poly(3-hexylthiophene) in propylene carbonate (PC) and aqueous solutions of 0.1 M LiClO4. The redox process of the polymer in propylene carbonate is accompanied by significant changes of the polymer thickness. This phenomenon is discussed in terms of polymer swelling and structural reconfiguration of the film, markedly delayed with respect to the charge transfer between the polymer and the substrate and exchange of the ions between the polymer matrix and the electrolyte solution. A large hysteresis for expansion and shrinking of the film is observed. In aqueous solutions, the hydrophobicity of alkyl chains limits the transfer of water molecules into the film and, in consequence, hinders the film reconfiguration. The thickness changes observed during polymer doping−undoping in water are about six times smaller than those observed in PC solution.