The methodology was employed to characterize unknown carboxylic acids in six Danish North Sea crude oils. Seven carboxylic acid standards were spiked to crude oils for quality control purposes to monitor the identification process. The number of full MS chromatographic fraction/peaks was averagely 8 -10 fractions, depending on detected AIF chromatographic peaks (S/N > 10) at m/z 169 and 171. All identified acids were merged to evaluate presented carboxylic acids in crude oil samples. Due to irrelevances when merging averaged full-MS LC peaks, the m/z intensity of carboxylic-BNMA was not used for data analysis, but the number of assigned formulas was. Generally, 125 to 380 compounds are identified containing carboxyl functional group Naphthenic acids with two rings appear to be the most common carboxylic acids after aliphatic acids. Also, the results accord with previous observations that acid constituents in similar crude oils range from aliphatic carboxylic acids to acids with aromatic rings and multiple functional groups. The CHNO class with dominant NxOy compounds has been commonly suspected as nitrogen-containing carboxylic acids bearing pyrrole and pyridine cores. The N1O2 subclass was commonly observed with DBE above ten could be monocarboxylic carbazoles and benzocarbazoles, caused by biodegradation processes. However, most of the observed nitrogen-containing acids (CHNO) by the current method had low DBE values up to 7, suggesting the presence of pyrrole, indole, and indoline carboxylic acids. Possibilities for O2S1 compounds with DBE < 4 could be sulfide or thiophene carboxylic acids.