Electrochemical response of regio-random and regio-controlled poly(3-hexylthiophene), P3HexTh, was investigated by cyclic voltammetry. P3HexTh under-went electrochemical oxidation at about 0.4 V vs. Ag+/Ag in a THF solution, and the peak anode electric current, i(pa) was proportional to the sweeping rate v; i(pa) = const x v(1/2). These data indicated that diffusion of the P3HexTh molecule in the solution was important to determine i(pa). Application of a Matsuda's equation with assumptions gave a diffusion coefficient, D, of about 1 x 10(-7) cm(2) s(-1) at molecular weight of about 5000, and the D value steeply decreased with increase in the molecular weight. (C) 2004 Elsevier Ltd. All rights reserved.