The carbon cycle is a key control for the Earth's climate. Every year rivers deliver a lot of organic carbon to coastal seas, but we do not know what happens to this carbon, particularly in the tropics. We show that rivers in Borneo deliver carbon from peat swamps to the sea with at most minimal biological or chemical alteration in estuaries, but sunlight can rapidly oxidise this carbon to CO₂. This means that south-east Asian seas are likely hotspots of terrestrial carbon decomposition.