Naphthenic acids (NAs), present in a typical Brazilian acid crude oil and its thermal degradation products, were investigated using two separation methodologies: solid-phase extraction (SPE) and liquid-liquid extraction (LLE). Fractions produced were characterized by proton nuclear magnetic resonance spectroscopy (H-1 NMR) and negative-ion-mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI(-)-FT-ICR MS). Of the NAs extraction methods studied, SPE was more efficient than LLE. Further, ESI(-)-FT-ICR MS results showed that the SPE method with eluent phase variation allowed for the detection of a larger amplitude of NAs compounds (m/z 200-1200), reducing the occurrence of ion suppression on the NAs of higher average molecular weight (M-w) distribution. It was noted that the aromaticity or double bond equivalent (DBE) of these produced collective fractions as well as their M-w values increased as a function of the polarity of the extraction system (DCM -> DCM:MeOH:FA). Also, H-1 NMR analysis revealed the alkyl predominance evidenced by the presence of high Hp content in fractions, suggesting that the NAs compounds have long and unbranched chains. The behavior of NAs species during the thermal degradation process was also evaluated, and the results showed their presence in only five SPE extracts out of six, containing different M-w values (M-w = 366, 417, 531, 662, and 836 Da). This suggests that, in the last SPE fraction (named SF6: m/z 700-1150, carbon number of C-52-C-72, and DBE = 0-15; detected only in virgin crude oil), the NAs were selectively cracked during the thermal degradation process.