A simple and sensitive method for the determination of nanomolar levels of hydrogen peroxide (H(2)O(2)) in seawater has been developed and validated. This method is based on the reduction of H(2)O(2) by ferrous iron in acid solution to yield hydroxyl radical (OH) which reacts with benzene to produce phenol. Phenol is separated from the reaction mixture by reversed phase high performance liquid chromatography and its fluorescence intensity signals were measured at excitation and emission of 270 and 298nm, respectively. Under optimum conditions, the calibration curve exhibited linearity in the range of (0-50)x10(3)nmolL(-1) H(2)O(2). The relative standard deviations for five replicate measurements of 500 and 50nmolL(-1) H(2)O(2) are 1.9 and 2.4%, respectively. The detection limit for H(2)O(2), defined as three times the standard deviation of the lowest standard solution (5nmolL(-1) H(2)O(2)) in seawater is 4nmolL(-1). Interference of nitrite ion (NO(2)(-)) on the fluorescence intensity of phenol was also investigated. The result indicated that the addition of 10micromolL(-1) NO(2)(-) to seawater samples showed no significant interference, although, the addition of 50micromolL(-1) NO(2)(-) to the seawater samples decreases the fluorescence intensity signals of phenol by almost 40%. Intercomparison of this method with well-accepted (p-hydroxyphenyl) acetic acid (POHPAA)-FIA method shows excellent agreement. The proposed method has been applied on-board analysis of H(2)O(2) in Seto Inland seawater samples.