A catalytic-kinetic method for the determination of selenium is proposed. Brilliant cresyl blue is reduced by sodium sulphide to form a colourless product. The reaction is accelerated by ultra trace amounts of Se(IV), and can be followed by measuring the decrease in absorbance of the dye at 630 nm and 30°C. selenium can be determined in the range of 0.0001 – 0.500 μg.ml−1with a limit of detection of 3 × 10−5μg.ml−1. The relative standard deviations are 2.5, 2.2 and 2.0% for 0.10. 10.0, and 100.0 ng.ml−1of Se(IV), respectively. This method was successfully applied to the determination of selenium in waste water.