Bromate levels ranging from 20 to 30 parts per billion (ppb) have recently been observed following the ozonation of Lake Michigan water. These bromate levels were a result of high bromide-to-bromate conversion, e.g., greater than 85% conversion (by weight) at times. This paper describes high bromide-to-bromate conversion during the ozonation of Lake Michigan water and highlights the following: factors influencing bromate formation; control of bromate formation; and, the challenge of accurate bromate analysis. Includes 23 references, table, figures.