Normal electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry in negative-ion mode has been used to compare the composition of Russian and North Sea crude oils and their eight different distillation fractions (160-210, 210-260, 260-310, 310-360, 360-410, 410-460, 460-510, and 510-560 °C). This is the first time that the polar compound distribution, especially species containing O, O 2 and N atoms, has been studied as a function of temperature. The results obtained are consistent with measured total acid number and nitrogen values; the North Sea crude oil had higher acid but lower nitrogen content compared to the Russian crude oil. The influence of high acid content on the ionization efficiency of other polar species (O and N) in oil samples is also presented for the first time.