The ultrahigh mass resolving power (> 350000) and mass accuracy (< 1 ppm) of electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enable resolution and identification of most of the thousands of polar heteroatomic species in petroleum distillates. Here, we compare elemental compositions of components of a 1:1 mixture of light cycle oil and refined chemical oil before and after each of three catalytic hydrotreatments over NiMo: single-pass at 690 degrees F, double-pass at 690 and 725 degrees F, and double-pass for a reactor partly clogged with coke. The effects of catalysis are reflected by changes in the relative abundances of various heteroatomic classes (NnOoSs), aromaticity (rings plus double bonds, or double bond equivalents, DBE) distributions for species of a given class, and carbon number (i.e., alkylation) distributions for species of a given class and DBE. Apart from expected changes (e.g., selective removal of heteroatom-containing species, reduction of aromatic species), some surprising results (e.g., increase in relative abundance of the most highly aromatic species, increase in carbon number for a given class and DBE, etc.) can be explained by partial saturation of aromatic rings and preferential removal of saturated rings, but incomplete cleavage of carbon-nitrogen bonds, in accord with a theoretical catalytic model. Incomplete hydrotreatment is shown to introduce new heteroatomic compounds not found in the original sample. Detailed elemental composition comparisons of the type shown here provide a new and rational basis for optimizing parameters for hydrotreatment of hydrocarbon fuel streams and evaluation of catalyst performance. Identification of which species increase or decrease in abundance and (especially) which new species are produced should greatly improve understanding of differential catalytic efficiency for removal and/or conversion of potentially all of the chemical constituents of an oil feedstock.