The photoluminescence (PL) spectra of poly[2,5-bis-(tri-ethoxy)-1,4-phenylene vinylene] (BTEO-PPV) films are blue-shifted with increasing thermal annealing temperature. It is known from the UV-vis absorption spectra that thermal annealing decreases the conjugation length of the polymer. For BTEO-PPV films, unlike with MEH-PPV films, the symmetric triethoxy side groups further block aggregation of the polymer chains. The absorption Fourier transfer infrared spectra showed that thermal annealing in atmosphere destroyed the chain structure of BTEO-PPV film by thermal oxidation to form aldehyde groups, which resulted in low PL efficiency of the annealed films. (c) 2005 Wiley Periodicals, Inc.