During ozonisation of bromide-containing waters, bromate is formed, which is classified as a potential cancerogen. An overview of the situation in Switzerland has shown that, as a rule, waterworks that use ozone do not exceed the new tolerance value of 10 µg/l of bromate. There is a characteristic relationship between bromate formation and the ozone exposure in a particular water type. This can be used to estimate the integral ozone exposure from bromate formation which allows to assess the disinfection. This new concept will be illustrated by means of two examples.