The optical properties of spin-cast poly(3-butylthiophene) (P3BT) films have been determined from 1.2 to 5.0 eV by spectroscopic ellipsometry. Dielectric-function spectra epsilon = epsilon(1) + i epsilon(2) with well-resolved vibronic shoulders were extracted by multilayer calculations from the ellipsometric data. The effect of annealing on 8 is also reported. The epsilon spectra of the annealed films show an increase of oscillator strength and a redshift of the main peak, which indicates the growth of an ordered structure due to self-assembly of the polymer chains. This conclusion is confirmed by X-ray diffraction. Photo-oxidative degradation is also investigated. We found that both light and oxygen are essential in causing degradation. The changes in conjugation length of degraded polymers on exposure are directly reflected in sand can be parameterized in terms of the decreasing value of the main peak in epsilon(2). We also noticed that the film thickness increases on exposure, suggesting decreased film density. (C) 013 Elsevier B.V. All rights reserved.