An optrode for Hg2+ ions based on a Nafion-1-(2-thiazolylazo)-2-naphtol (TAN) composite thin film has been developed. This optrode has a wide linear response range of 1-75 microM (pH 6) of Hg2+ ions with a limit of detection of 0.05 microM (10 micrograms l-1), which is sufficiently adequate for many environmental monitoring applications. The reflectance signal response shows a close correlation with the theoretical model derived. The response time of the optrode thin film was within 5-7 min to reach 95% of the final signal, depending on the concentration of Hg2+ ions. The selectivity of optrode to Hg2+ ions in phosphate buffer is good, with Co2+ and Ni2+ ions as the main interferences.