A comprehensive investigation of oil composition is important for the petroleum industry as well as the understanding of natural science. Ketones commonly exist in petroleum; however, as a result of the low concentrations and limitations inherent in the analytical technique, the molecular composition of these compounds is rarely studied. In this study, ketones in a source rock were characterized by gas chromatography-mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and Orbitrap mass spectrometry. Ketones were derivatized with the Girard T reagent under weakly acidic conditions to enhance their detestability by electrospray ionization mass spectrometry (ESI MS) analysis. Atmospheric pressure photoionization mass spectrometry (APPI MS) was also used for the molecular characterization of ketones without chemical derivatization. The characterization techniques were complementary to each other, and the results were consistent in general. The chemical derivatization followed by positive-ion ESI MS analysis is suitable for the analysis of ketones in a trace amount, but it had discrimination on high double bond equivalent (DBE) species (DBE >= 9). Direct analysis with positive-ion APPI MS showed comparable ionization efficiency throughout the DBE range and could distinguish aliphatic ketones from aromatic ketones according to different ionization pathways.