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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 (¹H 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 …