Molecular structure of heavy petroleum could be investigated by the composition of its ruthenium ion catalyzed oxidation (RICO) products. However, the interpretation of the results was not comprehensive due to the limited compositional information obtained solely by gas chromatography (GC) analysis. In this study, a semiquantitative method based on electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was established and applied for the molecular characterization of RICO products. Thousands of polar compounds were detected by negative-ion ESI FT-ICR MS in the RICO products of the Canadian oil sands bitumen derived asphaltenes. Besides alkyl carboxylic acids, naphthenic acids with one to five naphtha rings, nitrogen- and sulfur-containing carboxylic acids, and acidic compounds with multioxygen atoms were observed. The upper carbon number limit of alkyl moieties connected to the aromatic cores of the asphaltenes was found up to 60, which is much higher than the results derived from GC analysis. Normal and isomer alkyl carboxylic acids, as well as naphthenic acids, were quantitatively analyzed separately. The quantitative results of alkyl carboxylic acids from ESI FT-ICR MS agreed well with the GC results. The FT-ICR MS results indicate that additional compositional information could be obtained from RICO analysis. In addition, the method is instructive for the development of quantitative analysis technology for petroleum molecular characterization based on ESI FT-ICR MS.