The Dar crude oil produced in Sudan was distilled into several fractions.The heteroatom class species in crude and its fractions were characterized by the negative-ion ESI FT-ICR MS.The major emphasis was put upon the study on the O2 class species (petroleum carboxylic acids).The test results revealed that aliphatic acids and monocyclic naphthenic acids accounted for a large proportion in petroleum carboxylic acids of the Dar crude.The relative abundance of aliphatic acids and monocyclic naphthenic acids decreased with an increasing boiling point of fractions.Meanwhile,the relative abundance of bicyclic and tricyclic naphthenic acids increased at first and then decreased,whereas the relative abundance of petroleum carboxylic acids with higher double-bond equivalence (DBE) values increased.The high abundance of aliphatic acids in the Dar crude and its fractions covered the carbon numbers in a range of 16 to 18 which had important geochemical significance.The O2 class species were distributed in a broad range of DBE values and carbon numbers with increasing boiling points of fractions.