AbstractHydrogen peroxide (H₂O₂) is present ubiquitously in marine surface waters where it is a reactive intermediate in the cycling of many trace elements. Photochemical processes are considered the dominant natural H₂O₂ source, yet cannot explain nanomolar H₂O₂ concentrations below the photic zone. Here, we determined the concentration of H₂O₂ in full depth profiles across three ocean basins (Mediterranean Sea, South Atlantic and South Pacific Oceans). To determine the accuracy of H₂O₂ measurements in the deep ocean we also re-assessed the contribution of interfering species to ‘apparent H₂O₂’, as analysed by the luminol based chemiluminescence technique. Within the vicinity of coastal oxygen minimum zones, accurate measurement of H₂O₂ was not possible due to interference from Fe(II). Offshore, in deep (>1000 m) waters H₂O₂ concentrations ranged from 0.25 ± 0.27 nM (Mediterranean, Balearics-Algeria) to 2.9 ± 2.2 nM (Mediterranean, Corsica-France). Our results indicate that a dark, pelagic H₂O₂ production mechanism must occur throughout the deep ocean. A bacterial source of H₂O₂ is the most likely origin and we show that this source is likely sufficient to account for all of the observed H₂O₂ in the deep ocean.