A bulk liquid membrane system has been used to preconcentrate and separate the copper ions present in a seawater sample prior to its analysis by flame atomic absorption spectroscopy. The liquid membrane comprised an organic solution containing di-(2-ethylhexyl) phosphoric acid (DEHPA) in kerosene. The preconcentration system was optimized by studying the influence of the acidity of the sample and the receiving solution, the DEHPA concentration in the organic membrane, and the stirring rate. Under optimum conditions, the preconcentration yield for real samples was 87.0%. The average relative error of the determination was 3.6%, while relative standard deviation averaged 7.8%. The reliability of the new method was confirmed by analyzing several real samples of seawater, analyzed previously by a well-established technique.