A system and process for de-halogenating ballast water before releasing the ballast water from the vessel. In one embodiment, the system comprises a means for measuring the halogen content of the ballast water, a reducing agent source in fluid communication with the ballast water, and a means for controlling the amount of reducing agent supplied to the ballast water. In one aspect, the means for measuring the halogen content comprises one or more oxidation/reduction potential analyzers. In another embodiment, the system comprises one or more hypochlorite electrolytic cells for generating hypochlorite to treat the ballast water. One embodiment of the process for de-halogenating ballast water comprises measuring the oxidation/reduction potential of the ballast water and adding one or more reducing agents to the ballast water to de-halogenate the ballast water in response to the measured oxidation/reduction potential. In one aspect, the oxidation/reduction potential is modulated so that excess reducing agent is present in the ballast water.