We assessed natural degradation of fuel oil in three marshes from Galicia (Spain) affected by the Prestige oil spill (Baldaio, Earizo, and Muxia). Soil samples collected from polluted and unpolluted areas on four different dates were used to determine total petroleum hydrocarbon content and fuel-oil components. Natural degradation was monitored by analysing changes in the proportion of saturated hydrocarbons, aromatics, asphaltenes and resins in the soils, and also by evaluating the degree of depletion of saturated hydrocarbons on each sampling date. We additionally assessed the phytoremediation potential of Lolium perenne, L. Convolvulus arvensis L and Raphanus raphanistrum L All marsh soils exhibited natural degradation of saturated and aromatic hydrocarbons to between 85 and 95% in most cases. In contrast, asphaltenes and resins were degraded to a lesser extent (viz 64-76% in Barizo 1. Muxia and Traba; 39-44% in Baldaio; and only 12% in Barizo 2,where flooding by the river continues to introduce balls of fuel oil into the soil). Monitoring analyses revealed natural degradation to be dependent on the thickness of the pollutant layer. Field plots sown with L perenne L exhibited no significant differences in fuel-oil degradation from untreated plots. (C) 2008 Elsevier B.V. All rights reserved.