Oxidant decay and bromate formation were studied under light and dark conditions in 5, 15 and 30% artificial sea-water and approximately equal to 5% natural estuarine water following ozonation or chlorination. For both oxidants, light exposure accelerated the residual oxidant decay rates which were inversely related to sample salinities in artificial sea-water. Significant quantities of bromate were produced in light-exposed, chlorinated samples with an initial residual oxidant concentration of approximately equal to 70 micrometers approximately equal to 5 mg/l as total residual chlorine but not at lower residual oxidant concentrations or in non-photolyzed samples. No bromate was formed in any of the chlorinated natural estuarine water samples. Bromate production was much greater in ozonated samples than in chlorinated ones and was formed in two distinct stages. Photolytic bromate formation decreased with increasing bromide concentration in both chlorinated and ozonated artificial sea-water. Bromate formation was completely inhibited in the presence of NH3-N and estuarine sediment. The same free radical mechanism is proposed for both ozone-induced and photolytic-induced bromate formation in artificial sea-water. (Copyright (c) 1981 Pergamon Press Ltd.)