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Many river systems of the world are super-saturated in dissolved CO2 (pCO2) relative to equilibrium with the atmosphere. Here we compare the coupled organic matter and pCO2 dynamics of the world’s two largest and most organic-rich river systems. The emerging data sets for the Congo River, joint with Amazon River data, enable us to begin to think more generally about the overall functioning of the world’s two largest river basins. Discharge is the primary control on POC and DOC export in both the Amazon and Congo Rivers. TSS yield from the Amazon is twentyfold greater per unit area than the Congo. However, despite low TSS concentrations, the Congo has a POC content approximately five times higher than the Amazon. The organic-rich character of both watersheds is reflected in the DOC export, with the Amazon exporting ~11% and the Congo ~5% of the global land to ocean flux (but care should be taken when describing estimates of TSS and carbon to the ocean since processing and sequestration in tidal and coastal areas can significantly alter TSS and carbon delivery, and last measuring stations are typically hundreds of kilometers from the sea). pCO2 in the Amazon mainstem range from 1,000 to 10,000 ppm, with floodplain lakes ranging from 20 to 20,000 ppm. Concentrations in the Congo are lower, with high values of 5,000 ppm. The elevated level of pCO2 even as far as the mouth of such major rivers as the Amazon and Congo, up to thousands of kilometers from CO2-rich small streams, poses a most interesting question: What set of processes maintains such high levels? The answer is presumably some combination of instream metabolism of organic matter of terrestrial and floodplain origin, and/or injection of very high pCO2 water from local floodplains or tributaries."