The molecular composition of five Medium Vacuum Gas Oils (MVGOs) was studied using 15T Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) coupled with Electrospray Ionization (ESI) and atmospheric pressure photoionization (APPI). Class and Double Bond Equivalence (DBE) distribution of the MVGOs were compared; non-basic, basic nitrogen and oxygen compounds were detected using ESI negative and positive mode while aromatic hydrocarbons and sulfur species were visible via APPI. Compositional distributions revealed definitive heteroatom class and type trends among the MVGOs, and provide evidence of the relationship between chemical composition of gas oils (in terms of aromaticity, maturity and biodegradation) and some of its macroscopic properties such as API gravity. Furthermore, the observed class relative abundances matched well with the sulfur and nitrogen content determined for the MVGO’s by conventional methodologies. These preliminary approaches are here reported for Colombian gas oils for the first time, and afford as insight that it is possible to obtain semi quantitative information from FT-ICR MS experiments.