Three high-acidity crudes,Dar,SZ36-1,and QHD326,were separated through distillation into several fractions,including diesel distillates,and VGOs.Samples were characterized by negative-ion ESI FT-ICR MS.The O 2 class species (petroleum carboxylic acids),which have a close relationship with corrosion of equipment caused by high-acidity crudes,were put in the focus of attention and were discussed in this paper.Monocyclic,bicyclic,and tricyclic naphthenic acids are the main types of petroleum carboxylic acids in naphthenic-base crudes (SZ36-1 and QHD326).But the main types of petroleum carboxylic acids in paraffinic-base crude (Dar) are aliphatic acids and monocyclic naphthenic acids.The O 2 class species in SZ36-1 and QHD326 are distributed in a wider range and have bigger DBE value (double-bond equivalence value) and carbon number than Dar.Bicyclic naphthenic acids have the highest proportion among petroleum carboxylic acids in diesel distillates,but monocyclic and tricyclic naphthenic acids also occupy a high proportion.Particularly,aliphatic acids in the diesel distillate of Dar still have high proportion among petroleum carboxylic acids.The distribution of petroleum carboxylic acids in VGO is basically identical.The bicyclic naphthenic acids assume the first place (about 25 m%),while the monocyclic and tricyclic naphthenic acids take the next place.The comparison of petroleum carboxylic acids in diesel distillates and VGOs has revealed that the molecules of carboxylic acids in VGOs are not only bigger but also more complicated. Three high-acidity crudes, Dar, SZ36-1, and QHD326, were separated through the distillation into several fractions, including diesel distillates, and VGOs. Samples were characterized by negative-ion ESI FT-ICR MS.The O 2 class species (petroleum carboxylic acids), which have a close relationship with corrosion of equipment caused by high-acidity crudes, were put in the focus of attention and were discussed in this paper. Monocyclic, bicyclic, and tricyclic naphthenic acids are the main types of petroleum carboxylic acids in naphthenic-base crudes (SZ36-1 and QHD326) .But the main types of petroleum carboxylic acids in paraffinic-base crude (Dar) are aliphatic acids and monocyclic naphthenic acids. The O 2 class species in SZ36-1 and QHD326 are distributed in a wider range and have bigger DBE value (double-bond equivalence value) and carbon number than Dar.Bicyclic naphthenic acids have the highest proportion among petroleum carboxylic acids in diesel distillates, but monocyclic and tricyclic naphthenic a Theids also occupy a high proportion. Particularly, aliphatic acids in the diesel distillate of Dar still have high proportions among petroleum carboxylic acids. The distribution of petroleum carboxylic acids in VGO is basically identical. The bicyclic naphthenic acids assume the first place (about 25 m %), while the monocyclic and tricyclic naphthenic acids take the next place. the comparison of petroleum carboxylic acids in diesel distillates and VGOs has revealed that the molecules of carboxylic acids in VGOs are not only bigger but also more complicated.