A novel, optical sensor, test strip has been developed for the spectrophotometric determination of trace amounts of uranyl ions, UO 2 2+ , based on immobilization of C.I. Mordant Blue 29 (Chromazurol S)/cetyl N,N,N-trimethyl ammonium bromide ion pair on a triacetyl cellulose membrane. Optimization of the sensor for the detection of low levels of uranyl ion is described. The test strip responded linearly to uranyl ions between 3.0×10 −7 and 6.0×10 −5 molL −1 ; the reproducibility of the sensor at a medium level of UO 2 2+ activity was ±0.55%. The optical sensor can be regenerated using 0.01molL −1 HCl or 0.01molL −1 NaF solution after 10min. The developed test strip was used in the determination of UO 2 2+ in ground water samples.