The use of ozone in water treatment encounters the bromate formation hindrance. If raw water contains bromide, bromate will be formed in the ozonation step. Because bromate is well known as a carcinogenic, it is very important to optimize the treatment process and minimize or eliminate the bromate formation. Response surface optimization methodology (three-variable, three-level experiment Box-Behnken design) was applied in a pilot plant to investigate the effect of operating conditions (pH, initial bromide concentration, and exposure time) on bromate ion formation. Results showed that most of the investigated water facilities have water that exceeds the international standards of bromate content. It also showed that bromate formation is maximum when the pH is close to 9. Bromate formation also favors higher initial bromide concentrations. However, further increase in initial bromide concentration will cause the bromate to be reduced to bromine (Br-2) by excess bromide (Br-). (C) 2011 Elsevier B.V. All rights reserved.