On January 30, 2000, an accidental oil release occurred from a fracture in the OSSA 11 pipeline where it crosses over the Rio Desaguadero in Bolivia, South America. This paper addresses the composition of the spilled oil and the unique weathering processes that occurred after the spill. Samples of oil, oiled sediment, water, and vegetation were collected from the riverbank and surrounding areas for approximately one year after the initial release. The samples were analyzed by gas chromatography with a flame ionization detector and by gas chromatography coupled with mass spectrometry to characterize the weathered product. Laboratory studies were also performed to evaluate the maximum extent of evaporation of the pipeline oil, and the extent of water solubility. Based on these chemical analyses, several conclusions were reached: There was a rapid and substantial loss of the benzene, toluene, ethylbenzene, and xylenes; (BTEX) and polycyclic aromatic hydrocarbons (PAH) oil fractions. The BTEX and PAH losses were due primarily to evaporation. The stranded residual oil consisted primarily of heavy, immobile hydrocarbons. Photooxidation of PAHs, including benzo(a)pyrene, was observed. A unique weathering mechanism was observed that selectively removed mid-range hydrocarbons (C20 through C39) from the bulk oil. The overall environmental risk of the spilled oil has been reduced due to the extensive weathering of the oil. (C) 2002 Published by Elsevier Science Ltd.