The evaluation of colorimetric films for the detection of low concentrations of gaseous ammonia in a waveguide configuration is presented. They were composed of a pH indicator selective to NH3 embedded in a polymeric matrix and deposited on an optical waveguide to improve the sensitivity. Since the film aims at being integrated onto flexible plastic foil, a plasticizer was added to allow bending of the film without cracking. The different components were dissolved in a proper amount of solvents to obtain spincoatable sub-µm thin films. They were successfully deposited onto glass waveguides to evaluate their optical properties and gas sensing abilities. The influences of the matrix composition and the pH indicator used on their gas sensing characteristics are presented. All films exhibited a very good selectivity and cross- selectivity towards NH3. A matrix composed of bromophenol blue and PMMA exhibited the best sensing properties. The adequate choice of plasticizer has shown a significant reduction on the sensitivity of the film to humidity at the cost of a slower kinetics of reaction. Concentrations as low as 0.25ppm of NH3 were easily measured and the theoretical limit of detection of our sensing device was evaluated to be of 2 ppb.