Waveguiding and refractive index measurements were performed at a number of wavelengths in the red and near infrared region of the spectrum by grating coupling into a series of polythiophenes, poly[3-tetradecylthiphere] (PT14), poly[3-decylthiophene] (PT10) and poly[3-hexylthiophene] (PT6) films. The dispersion in the refractive index was compared to the Kramers-Kronig analysis of the absorption spectra. A decrease of the refractive indices with increasing number of side chains was found due to a volume dilution effect of the optically active main chain.