Abstract: In a phthalate buffer medium of pH 2,the catalytic action of Se(Ⅳ) on the reductive color-fading of xylenol orange by sodium sulfide was studied. It was found that the catalytic reaction was a zero order reaction,the apparent constant of reaction rate was found to be 7.67×10-5mol·L-1·s-1,and the apparent activation energy was 50.09 kJ·mol-1. Linear relationship between values of ΔA (difference of absorbances between the catalytic and non-catalytic reactions) and concentration of Se(Ⅳ) was obtained within 0.12 mg·L-1 of Se(Ⅳ),with a detection limit of 2.66×10-5g·L-1. The proposed method has been applied to the determination of traces amounts of Se(Ⅳ) in water samples.