Optical properties in terms of the complex dielectric function of thin films of poly(3-hexylthiophenes) on gold substrates have been measured with spectroscopic ellipsometry in the photon energy range 1.5-3.5eV. With line-shape analysis using a modified Lorentzian model, the broad absorption band around 2.25 eV can be resolved in three resonances, and the changes in the line-shape parameters due to degradation, heat treatment and the dependence of film thickness were studied. Degradation is observed as a loss of total oscillator strength and broadening of the central resonance in the absorption band, while heat treatment mainly results in a redistribution of oscillator strengths. The energy difference between the resonances were found to be constant when the absorption coefficient is analysed, but not if the dielectric function is analysed. As the latter quantity is more directly related to the density of states distribution, this indicates that the previously suggested vibronic model for the absorption band is incomplete.