The diminishing clean oil reserve is driving the search for new or improved ways to reduce the level of NSO-containing species found in high abundance in heavy crude oils. Hydrotreatment is the currently preferred technique to remove those polar species. Unfortunately, nitrogen-containing compounds cause coke formation on the surface of the hydrotreatment catalyst, leading to partial or complete deactivation. Here, positive- and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) identify those nitrogen compounds that resist hydrotreatment. ESI preferentially ionizes polar (e.g., heteroatom-containing) species: basic molecules are detected as positive ions and acidic/neutral molecules as negative ions. FT-ICR MS resolves thousands of species in a single mass spectrum, allowing for unambiguous determination of elemental composition, CcHhNnOoSs, for identification of compound "class" (numbers of N, O, S heteroatoms, "type" (rings plus double bonds), and carbon number (revealing the extent of alkylation). We find that hydrotreatment-resistant compounds typically contain a single nitrogen atom, both pyridinic benzalogs and pyrollic benzalogs. Compounds with more than one heteroatom, such as O-x, NxOy, NxSy and N-x, are partially removed. Compound classes with lower double bond equivalents or fewer CH2 groups are preferentially removed. Species that contain OxSy are fully removed by hydrotreatment. (C) 2006 Elsevier Ltd. All rights reserved.