There is as much carbon in dissolved organic material in the oceans as there is CO2 in the atmosphere(1), but the role of dissolved organic carbon (DOC) in the global carbon cycle is poorly understood. DOC in the deep ocean has long been considered to be uniformly distributed(2,3) and hence largely refractory to biological decay(4), But the turnover of DOC, and therefore its contribution to the carbon cycle, has been evident from radiocarbon dating studies(5,6). Here we report the results of a global survey of deep-ocean DOC concentrations, including the region of deep-water formation in the North Atlantic Ocean, the Circumpolar Current of the Southern Ocean, and the Indian and Pacific oceans. DOC concentrations decreased by 14 micromolar from the northern North Atlantic Ocean to the northern North Pacific Ocean, representing a 29% reduction in concentration. We evaluate the spatial patterns in terms of source/sink processes. Inputs of DOC to the deep ocean are identifiable in the mid-latitudes of the Southern Hemisphere, but the mechanisms have not been identified with certainty.