Au/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH- PPV) composite films were prepared by electrodeposition of Au nanoparticles using two deposition conditions followed by the spin coating of MEH-PPV. In spite of the similar average particle size, surface plasmon bands and the bandwidth of the two types of Au nanoparticles were different from each other due to the particle density. The electrochromic coloration of Au/MEH-PPV films was modified when compared to pure MEH- PPV films, in that a new coloration band originating from the surface plasmon band of Au nanoparticles appeared. Depending on the types of Au nanoparticles, the electrochromic coloration of the composite films was also strongly influenced. (c) 2006 The Electrochemical Society.