a b s t r a c t A waveguide interferometer based free-chlorine sensing technique has been developed. A polymer film for a specific free chlorine binding was designed, synthesized and applied on the surface of a waveg-uide. The material is based on cyanuric acid moieties along each repeating unit covalently tethered to poly(norbornene)s. Chlorine sensing was accomplished by measuring the refractive index change of the polymer, as a result of the reaction between cyanuric acid and free chlorine, interferometrically by the evanescent field extended above the waveguide surface. The free chlorine binding to cyanuric acid is reversible and a linear calibration curve from 0.1 to 10 mg L −1 of HOCl concentration was obtained with the level of detection (LOD) and level of quantification (LOQ) of 0.047 and 0.328 mg L −1 of HOCl, respec-tively. A free chlorine measurement with less interference from combined chlorine than DPD based colorimetric method was developed as a result of the different sensing responses of free and combined chlorine. Free chlorine residual was measured in samples collected from tap water and poultry process-ing waters by both optical sensor and DPD-based colorimetric method. Good agreement between both methods was observed although the levels for free chlorine measured by the optical sensor are system-atically lower than the readings obtained from the DPD method. The difference might be the result of the interference from combined chlorine during the DPD measurement.