Many rivers systems of the world are super-saturated in dissolved CO2 (pCO2) relative to equilibrium with the atmosphere — why? Here we compare the coupled organic matter and pCO2 dynamics of the world’s two largest river systems, the Amazon and Congo, where data sets enable insights into the overall functioning of the respective basins. Discharge is the primary control on particulate (POC) and dissolved organic carbon (DOC) export in both the Amazon and Congo Rivers. Total suspended sediments (TSS) yield from the Amazon is twenty times 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, based on measurements from the last discharge gauging stations. But care should be taken when describing estimates of TSS and carbon to the ocean. Processing and sequestration in tidal and coastal areas can significantly alter TSS and carbon delivery, and last discharge gauging stations are typically hundreds of kilometers from the sea. pCO2 in the Amazon mainstem ranges from 1,000 to 10,000 μatm, with floodplain lakes ranging from 20 to 20,000 μatm. Concentrations in the Congo mainstem are lower, with maximum values of ~5,000 μatm observed. 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.