A catalytic kinetic method is described 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 bromate by hydrazine dihydrochloride in acidic media. The generated bromine decolorized methyl orange (MO) and the reaction was monitored spectrophotometrically at 507nm as a function of time. The initial rate and fixed time methods were adopted for the determination and speciation of inorganic selenium. Under two optimum conditions, the calibration graphs are linear in the range 0-126.3 and 0-789.6 mu gL(-1) of Se(IV) for the initial rate method and 0-315.8 and 0-789.6 mu gL(-1) of Se(IV) for the fixed time method. The detection limits were 1.3 and 14.7 mu gL(-1) for the initial rate and fixed time methods, respectively. The proposed methods were validated statistically and through recovery studies in environmental water samples. The relative standard deviation in the determination of 31.6-94.8 mu gL(-1) of Se(IV) and Se(VI) was less than 6%. Analyses of standard reference materials for selenium using initial rate and fixed time methods showed that the proposed methods have good accuracy. Se([V),Se(VI) and total inorganic selenium in environmental water samples have been successfully determined by this method after selective reduction of Se(VI) to Se(IV). (C) 2008 Elsevier B.V. All rights reserved.