Electrospray ionization mass spectrometry (ESI−MS) is becoming a common method for the analysis of petroleum-based chemicals. Here, we have demonstrated the usefulness of this method for the analysis of polar resin fractions of crude oil, fuel oil, and diesel. The mass spectra of crude oil resins can be used to make comparisons between the constituents present in these oils. Tandem mass spectra of one crude oil sample identified a major constituent to be alkylated carbazoles. The analysis of resins from a diesel sample showed the presence of quinoline compounds, along with oxygen or sulfur heterocycles and alkyl carbazoles. The effect of weathering on the composition of petroleum resins was studied by comparing the average number molecular weights generated from mass spectra.