Aliphatic hydrocarbons in oil samples collected from April to October 2008 have been analyzed to evaluate the short-term weathering effect on these spilled oil residues and to obtain measures of the practical depletion level of specific compound groups. Weathering processes have changed the chemical composition of these residual specimens of spilled oil remarkably, showing the high incidence of natural weathering processes on the spilled crude oil 5∼10 months after the accident. Other weathering processes that are indicators of more advanced degradation processes did not show significant variation during the surveyed periods.