The exposure to pentane vapour of poly(3-alkylthiophene)s with different alkyl chains produces a modification in the optical properties of the thin films. The treatment induces a red-shift of the electronic absorption maximum and the appearance of vibronic structure in the UV spectra. We have followed these changes with IR and Raman spectroscopies to monitor the modification of the backbone conjugation, and with X-ray diffraction to evidence the structural order of the chains.