Analysis of total dissolved boron and boron isotopes for interstitial waters from recent Ocean Drilling Program drill cores in two accretionary prisms indicates an efficient return to the ocean of exchangeable boron of subducted sediments via pore-fluid expulsion. This source of boron helps to balance the ocean sinks, but a complete balance of the ocean budget requires deeply subducted boron to be returned efficiently to the oceans via explosive arc volcanism.