Abstract Fibre-coupled optical sensors for chemical and biological species are important for process control, environmental control and pollution detection, for example. Here, we report on an integrated-optic ammonia sensor, which is based on evanescent-field absorption. The sensitive element of this sensor is a strip waveguide, fabricated by field-assisted ion-exchange, coated with an immobilized indicator dye. The sensor has a short response time and long lifetime. We describe the experimental arrangement that has been built up for the characterization of different integrated-optic sensor elements. Spectral response characteristics, response times and the generation of reference signals are reported in detail. Furthermore, we show the simple temperature dependence, the humidity independence and the very low cross-sensitivity of this ammonia sensor.