The attenuated total reflection (ATR) and emission light properties utilizing surface plasmon (SP) excitations were measured for the electrochemical change of poly(3-hexylthiophene-2,5diyl) (P3HT) thin films in-situ. The SP emission light could detect the SP excited by molecular luminescence of P3HT. The ATR and SP emission light properties were observed for the doped-dedoped states of P3HT thin film. The ATR and SP emission light properties were remarkably changed with the electrochemical doping and dedoping. The SP emission light also decreased by decrease of the molecular luminescence of P3HT by doping. For the dedoped-state P3HT thin film, SP emission light also increased by increase of the molecular luminescence. The SP emission light excited by molecular luminescence can be controlled by the control of doping-dedoping state. (C) 2009 Elsevier B.V. All rights reserved.