Chlorination of seawater is still considered as one of the most effective and least expensive process to control biofilm formation (micro-fouling) that may block the heat exchangers or development of molds (macro-fouling) in industrial pipelines. This process is used in many industrial sites around the world in order to use this water for cooling purposes (petrochemical and steel industries) or for warming (LNG terminals for the liquefaction of gases). This process has also been selected by the International Maritime Organisation as one of the methods to be used for the control and management of ships ballast water before discharge into the sea. Whatever type of industries, whether on land or at sea, chlorination is conducted either by dissolving chlorine gas, or by adding sodium hypochlorite, or -even in the case of use of seawater- by electrolysis at doses between 0.5 and 1.5 mg L-1 (expressed as Cl 2) on water taken from the environment at rates of several hundred m3 /s. It can be conducted in continuous mode at low dose to stop the settlement of larvae, reducing their growth and kill the long term or intermittently then when the heat exchanger fouling reaches a certain threshold. The major concern related to the use of this process is, to some extent, dumping of chlorine in the environment with known impacts on fauna and flora but above potential output of many halogenated compounds formed by complex reactions between the chlorine residual inorganic and organic constituents, natural and man present in the receiving environment. If the formation of chlorination by-products is well studied and referenced in the literature for water intended for human consumption and freshwater, little data exists on the nature and concentration levels and reactivity and fate of these byproducts in coastal waters, mostly based on the knowledge acquired by the reactivity of chlorine with organic matter in freshwater. However, it is known that chlorine (as Cl2 or HClO form) can rapidly react with the bromide ions found in seawater to form instantly hypobromous acid, compound known to be much more reactive than HClO with the organic matter. In marine environments, some research conducted to examine the impact of chlorination on the environment were limited to desalination plants for seawater and nuclear and thermal power plants, and on studies a single discharge point. They also focused on the occurrence of few compounds such as trihalomethanes and haloacetic acids, or even only on the overall toxicity of these releases without even find the nature of the present compounds. They are mainly limited to the measurement and identification of these compounds in the water compartment. FOS-SEA project aims at a better understanding of the nature of the by-products that can be generated during the chlorination processes when seawater is used as makeup water to determine the reaction schemes of their training and their become, to study their transfer in different environmental compartments and their dissemination, and finally to study their toxicity and their metabolites. The results of this research will assess the environmental impacts of releases of chlorinated water at sea and to acquire new knowledge that can help local and national regulatory authorities to formulate control strategies and guidelines for wise use and regulated the chlorination process.