The photochemical decomposition of hydrogen peroxide has been studied in surface waters collected on a transect from the Atlantic Ocean north of Puerto Rico through the eastern Caribbean Sea to the mouth of the Orinoco River. Absolute rates of photochemical decomposition were measured by adding 18O labelled H2O2 to the water samples and irradiating with a shipboard solar simulator. Photochemical decomposition was measurable at all station locations. Decomposition rates increased along the transect towards the Orinoco and were directly proportional to photochemical production rates, which also increased. Photodecomposition rates were also linearly related to the intensity of absorption of samples at 300 nm. Decomposition rates were generally ~5% of formation rates, indicating that photochemical decomposition is probably a minor sink for H2O2 in the mixed layer compared to biological decomposition. In all seawater samples, photocomposition led exclusively to 18O2, indicating that reaction with photochemically produced oxidants was the dominant reaction mechanism. The oxidizing species are not the best known photooxidants in seawater, singlet oxygen and OH and O2 radicals. Unidentified oxidants with formation rates ~5% of the H2O2 formation flux are required. In contrast, H218O2 decomposition in unfiltered samples of Orinoco River water led solely to H218O2, indicating reaction with a photochemically produced reductant was the dominant mechanism. Further experiments indicated that the reductant was probably Fe(II) produced by photoreduction of particulate Fe(III), a known process. This pathway would produce an equivalent flux of OH radical: Fe(II)+HOOH → Fe(III)+OH+OH-1. ¿ American Geophysical Union 1993