Hydrogen peroxide was determined in seawater at the Hiroshima Bay, the Seto Inland Sea under sunny conditions. Concentration of hydrogen peroxide was in accordance with the intensity of the solar radiation. Photochemical production under strong biological activity might be responsible for the pro files of the diurnal and seasonal variation of hydrogen peroxide concentration.The spatial distribution of hydrogen peroxide in the Seto Inland Sea gave a good correlation with salinity, suggesting that the photosensitizer to produce hydrogen peroxide was substances brought from river waters.Hydrated electron as a possible precursor of hydrogen peroxide in the photochemical reaction was generated in seawater collected at the Seto Inland Sea by means of irradiating the third harmonic wave of the YAG laser (355 nm: 60 mJ): The amount generated showed a positive correlation against total organic carbon (TOC) and dissolved organic carbon (DOC) in seawater. This correlation was greately improved by the addition of EDTA. The EDTA treatment suggests the existence of some species obstructed the hydrogen peroxide production. Also, this correlation coefficient was improved when tak ing fluorescent matter instead of TOC or DOC.