The blowout at the Ixtoc 1 offshore oil rig in the Gulf of Mexico (19°24' N, 92°12' W) produced a major oil spill. An estimated 140 million gallons of oil escaped before the well was finally capped1; this compares with a spill of 66 million gallons of oil from the tanker Amoco Cadiz2,3. In contrast to the rapid, close-to-shore release of the tanker's cargo, the Ixtoc 1 well released oil for 9 months into the open ocean where winds and currents dispersed the floating mousse (a light brown and orange water-in-oil emulsion) which had formed at the wellhead. Two and a half months after the blowout the western boundary current in the Gulf of Mexico4,5 had brought mousse to the American shore. An ultra-thin sheen also formed from the oil and was visible as a smoothness of sea-surface texture relative to the slightly rougher unpolluted waters. We report here that at a distance of 750-1,000 km from the well, Ixtoc 1 mousse occurred on sheen-covered sea surface primarily as gramme-sized pancakes and milligramme-sized flakes. Relative to pancakes, flakes were more dense, depleted in aliphatic compounds and enriched in polar compounds. Both particles were covered with a brittle 5-10 µm skin. Pancakes, skin, flakes and surface sheen seem to represent the four macroscopic component parts of a pelagic weathering cycle of Ixtoc 1 mousse.