Thermal reactions of hydrocarbons, which were loaded onto a river sediment, were investigated at 300 degrees C in an open and in a closed reactor. The results give evidence that the thermodesorption of PAHs and hydroxylated aromatics is accompanied by their chemical conversion to non-volatile products, which can be called charring. Quartz powder as an inert carrier does not effect such reactions. The rank in reactivity observed for charring of PAHs is consistent with their reactivity in radical reactions. The sediment ash is more active in hydrocarbon conversion reactions than the native sediment. This points to the mineral matrix of the sediment as the actual carrier of the catalytic activity rather than the sediment organic matter.