A sensitive gas chromatographic method has been established for the determination of bromate in ozonated and chlorinated water, and in sea water. With acidic conditions, bromate reacts with chloride to form bromine, which reacts with 2,6-dialkylphenol to form 4-bromo-2,6-dialkylphenol. The organic derivative was extracted with ethyl acetate after quenching remaining oxidants with ascorbic acid, and then measured by gas chromatography-mass spectrometry (GC-MS). The lowest detection limit and limit of quantification of bromate in drinking water were 0.02 and 0.07 μg/L, respectively, and the calibration curve showed good linearity with r²=0.998. The 32 common ions did not interfere even when present in 100-fold excess over the bromated ion. The accuracy was in a range of 102-106% and the precision of the assay was less than 6% in chlorinated and ozonated tap water, ozonated mineral water, and sea water. The method was sensitive, reproducible and simple enough to permit reliable analysis of bromate to the ng/L level in water.