The in situ optical absorption and fluorescence of poly(3-octylthiophene) (POT) upon electrochemical doping were studied. Fluorescence from the undoped POT was observed in the acetonitrile electrolyte. It was quenched by the electrochemical doping. The effect of a dopant molecule on the intensity change of the fluorescence is 7.5 times greater than the one for the optical absorption. Thus, the fluorescence quantum yield decreased after electrochemical doping. (c) 2006 Wiley Periodicals, Inc.