A new kind of optical sensor for monitoring dissolved ammonia uses rhodamines immobilized in thin membranes made from ethylcellulose, poly(vinyl chloride) or poly(vinyl acetate). The response to ammonia is the result of a change in the molecular structure of the rhodamine. On exposure to ammonia, the rhodamine is converted to a colorless and non-fluorescent lactone. As a result, fluorescence intensity is reduced. The polymer not only has a strong effect on the limits of detection, but also on the response time. Typical LODs are 0.1 μg ml−1 for ethylcellulose and poly(vinyl chloride), but as low as 10 μg ml−1 for poly(vinyl acetate). The last membranes provide highest sensitivity and lowest detection limits, but are not suitable for online monitoring of ammonia because they respond irreversibly. They may be applied, however, for “single shot” tests. Response times of the membranes are in the order of 2–5 min.