A thin film planar waveguide is studied for application as a chemical sensor for liquid-phase absorbance measurements. The waveguide Is comprised of a thin film of tantalum pent-oxide deposited on a glass substrate with a pair of diffraction gratings etched into the substrate surface. The buried grating couplers allow the launch and collection optics to be isolated from the liquid sample. The response to an absorbing dye and different refractive Index (RI) solutions is studied and compared to theoretical predictions. The sensor has an absorbance sensitivity equivalent to a 1 mm path length in a conventional transmission measurement. A method to reduce the Intensity changes due to solution RI is demonstrated.