The optical, electronic, and electrochemical properties of thin films of the electroactive polymer, poly(3-butylthiophene) (P3BT), were studied in a lithium hexafluorophosphate electrolyte. Upon extraction of n electrons per polymer formula unit, anions are taken up to balance the charge, forming a polymer cation salt, (P3BT) mn+(PF6 −) mn (m is the number of formula units in the polymer chain), whose state of charge (SOC) varies with n. An in situ ac impedance method was developed to determine the electronic conductivity of the polymer at different states of charge. On oxidation of the insulating neutral P3BT polymer, the conductivity increased by eight orders of magnitude. The electrochemical potential, electronic conductivity, and optical spectrum at each state of charge were correlated, producing a color index of these primary characteristics. This allowed determination of the potential and state of charge as a function of location in transparent polymer films during propagation of an oxidation front across the film and during passage of steady-state currents through the polymer film.