In power stations, the cooling effluents are chlorinated to avoid excessive biofouling. Yet, this disinfecting treatment leads to the formation of halogenated by-products, mainly trihalomethanes. So, there is a need for precise and accurate methods that allow trace levels determination of these compounds. A system that combines purge-and-trap and gas chromatography (with an electron capture detector) was used in this study. After careful choice of the experimental conditions, the performance of the system were evaluated. Precise and accurate determinations were obtained, allowing the determination of trihalomethanes in sea water samples chlorinated on site in three French coastal power stations. Bromoform was the predominant component formed, while traces of dibromochloromethane, chloroform and bromodichloromethane were also detected.