The dynamics of hydrogen peroxide (H2O2) was investigated from December 2007 to October 2008 in the Gulf of Aqaba, which in the absence of H2O2 contribution from biological production, rain and runoff, turned out to be a unique natural photochemical laboratory. A distinct seasonal pattern emerged, with highest midday surface H2O2 concentrations in spring-summer (30-90 nM) as compared to winter (10-30 nM). Similarly, irradiation normalized net H 2O2 formation rates obtained in concurrent ship-board experiments were faster in spring-summer than in winter. These seasonal patterns were attributed to changes in water characteristics, namely elevated spring-summer chromophoric dissolved organic matter (CDOM). The role of trace elements in H2O2 photoformation was studied by simultaneously measuring superoxide (O2-), Fe(II), and H2O2 formation and loss in ambient seawater and in the presence of superoxide dismutase, iron and copper. O2- was found to decay fast in the Gulf water, with a half-life of 15-28 s, primarily due to catalytic reactions with trace metals (predominantly copper). Hence, H2O2 formation in the Gulf involves metal-catalyzed O 2- disproptionation. Added iron moderately lowered net H2O2 photoformation, probably due to its participation in Fe(II) oxidation, a process that may also modify H2O2 formation in situ.