We report measurements of the dispersion in the linear optical properties (linear absorption coefficient and refractive index) of polydiacetylene para-toluene sulfonate (PDA–PTS) single crystal waveguide films prepared by thermal polymerization of the monomer free floating in solution and subsequent attachment of the film to the substrate. A maximum absorption coefficient >10 5 cm −1 at a wavelength of 620 nm is measured for polarization parallel to the polymer chains, in agreement with previous measurements. The refractive index parallel to the polymer axis is significantly lower in these thin films as compared to bulk PDA–PTS crystals, whereas the index perpendicular to the chains is comparable to that of bulk crystals. Sellmeier parameters describing the dispersion of the refractive index in the near-infrared spectral range are given.