The spillage of oil from the wreck of the supertanker Amoco Cadiz resulted in massive oiling of the littoral zone along 320 km of shoreline of the Brittany Coast. This study examined the fate of Amoco Cadiz oil which entered the intertidal zone. Biodegradation by indigenous microbial populations was an extremely important process in the weathering of the oil. Alkanes were preferentially degraded but branched alkanes and polynuclear aromatic compounds also were subject to microbial attack. The chemical evolution of the residual ‘hydrocarbon mixture’ within the littoral zone led to a relative enrichment of isoprenoid alkanes, naphtheno-aromatic and napthenic compounds, alkylated phenanthrenes, aromatic organic sulfur compounds e.g. dibenzothiophenes, hopanes and C 27−C 31 alkanes. These classes of compounds appear to be most resistant to biological and chemical weathering. Continued inputs of relatively unweathered oil, which may be preserved in subsurface anaerobic sediments, and pooling of oil becomes increasingly important with time, producing a patchy distribution of oil, which has been weathered to varying degrees. The presence of hydrocarbons within the littoral zone, as a highly weathered residual oil, appears to be a long lived (multi-year) phenomenon.