Two halide sensitive acridinium¯uorophores have been synthesised and immobilised in a hydrophilic copolymer. Thin ®lms of the copolymers swell in aqueous media allowing dye¯uorescence to be dynamically quenched by the di usion of halide ions. The new sensor ®lms have been characterised in terms of their sensitivity and selectivity towards halide and are compared to ®lms containing both bound and unbound Rhodamine B. The sensor ®lms are reversibly capable of determining halide ions at mildly alkaline pH with typical 90% response times of % 1±2 min. The sensor ®lms are cheap to produce, can readily be coated on disposable glass microscope slides or on the tip of a ®bre optic bundle and have a shelf life in excess of 2 years. #