In this study, laser desorption ionization (LDI) coupled to Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) was applied to study crude oils at the molecular level. Molecular ions were the major type of ion detected by (+) mode, and deprotonated and radical anions were the major ions observed by (-) mode LDI FTICR MS. N(1) and hydrocarbon classes were dominant in the class distribution plots obtained by (+) LDI FTICR MS, but other heteroatom classes, including O(x) and S(1), were abundant in plots obtained by (-) LDI FTICR MS. Detailed analysis of double-bond equivalence (DBE) vs carbon number plots revealed that LDI FTICR MS is more sensitive toward polyaromatic compounds than mono- or dicyclic-aromatic compounds. However, nonaromatic and aromatic O(2) compounds could be detected simultaneously. An abundance of nonaromatic O(2) compounds (presumably naphthenic acids) are correlated with total acid numbers, but O(2) compounds with condensed structures are not. Overall, this study shows that LDI FTICR MS can be successfully used to study crude oils at the molecular level.