A simple optical fibre chemical sensor based on a nafion - crystal violet film for the determination of gaseous ammonia in air is described. The film was prepared from a methanolic solution mixture containing 1.0 x 10(-3) mol L-1 crystal violet and 1.0 x 10(-3) mol L-1 nafion (in sulphonate groups) and determinations of ammonia concentration in air were achieved using reflectance measurements at 580 nm. Ammonia seems to react irreversibly with the film, since it cannot be released by flushing the film with dry air. However, a short pulse of humid air, followed by a stream of dry air can reverse the response of the film, providing a signal recovery greater than 90% at 580 nm. The film did not respond to carbon dioxide even at concentrations higher than 350 ppm, but it responds on exposure to 5 ppm of hydrogen hydrochloride and nitrogen dioxide. The influence of the relative humidity to the sensor response is also presented and response mechanism to ammonia in dry and humid air is suggested. A repeatability of 3.5%, expressed as the rsd of 6 measurements with a 20 ppm NH3 standard, was achieved. The sensitivity of the sensor is tuneable by altering the exposure time to ammonia gas. Detection limits of 1.40 ppm and 0.46 ppm and linear responses up to 50 and 10 ppm in dry air were achieved with exposure times of 30s and 5 min, respectively.191127133