Electrochemical response of regio-random and regio-controlled poly(3-hexylthiophene), P3HexTh, was investigated by cyclic voltammetry. P3HexTh underwent electrochemical oxidation at about 0.4 V vs. Ag + /Ag in a THF solution, and the peak anode electric current, i p a , was proportional to the sweeping rate v; i p a =constxv 1 / 2 . These data indicated that diffusion of the P3HexTh molecule in the solution was important to determine i p a . Application of a Matsuda's equation with assumptions gave a diffusion coefficient, D, of about 1x10 - 7 cm 2 s - 1 at molecular weight of about 5000, and the D value steeply decreased with increase in the molecular weight.