A novel catalytic kinetic method is proposed for the determination of Se(Ⅳ),Se(Ⅵ),and total inorganic selenium in water based on the catalytic effect of Se(Ⅳ) on the reduction of Celestine blue by sodium sulfide at pH 7.0 phosphate buffer.The fixed-time method was adopted for the determination and speciation of inorganic selenium.Under the optimum conditions,the two calibration graphs are linear with a good correlation coefficient in the range 2-20 and 20-200 μg.L-1 of Se(Ⅳ) for the fixed-time method at 30℃.The experimental and theoretical detection limits of the developed kinetic method were found to be 0.21 and 2.50 μg·L-1 for the fixed-time method (3 min).All of the variables that affect the sensitivity at 645 nm were investigated,and the optimum conditions were established.The interference effect of various cations and anions on the Se(Ⅳ) determination was also studied.The selectivity of the selenium determination was greatly improved with the use of the strongly cation exchange resin such as Amberlite IR120 plus as long as chelating agents of thiourea and thiosulphate.The proposed kinetic method was validated statistically and through recovery studies in natural water samples.The relative standard deviations (RSDs) for ten replicate measurements of 2,10,and 20 μg·L-1 of Se(Ⅳ) change between 0.35％ and 5.58％,while the RSDs for ten replicate measurements of 3,6,and 12 μg·L-1 of Se(Ⅵ) change between 0.49％ and 1.61％.Analyses of a certified standard reference material (NIST SRM 1643e) for selenium using the fixed-time method showed that the proposed kinetic method has good accuracy.The Se(Ⅳ),Se(Ⅵ),and total inorganic selenium in lake and river water samples have been successfully determined by this method after selective reduction of Se(Ⅵ) to Se(Ⅳ).