A vanadium-enriched fraction from the >700 °C resid of Cerro Negro heavy petroleum was analyzed by high-resolution, low-energy, electron-ionization mass spectrometry (low-eV HR/MS). This fraction, isolated in a prior investigation by liquid chromatographic methods, was known to contain 19 500 ppm vanadium, or 4.6 wt % of the total vanadium in the whole resid. On the basis of UV-visible absorption spectra, the concentration of porphyrinic vanadium in the fraction was previously estimated to be 11 000 ppm or 56.4 wt % of the total vanadium. Because the porphyrinic vanadium accounted for only about one-half of the total, an independent method was needed to characterize the nonporphyrinic vanadium in the fraction. Low-eV HR/MS with sample introduction by probe microdistillation was selected because of its capability for characterizing aromatic and polar compounds in low-volatility, high molecular weight mixtures. Etioporphyrins (C n H 2n-28 N 4 VO) with molecular weights ranging from 487 to 879 were found to be the most abundant vanadium-containing compounds in the fraction. Deoxophylloerythroetioporphyrins (C n H 2n-30 N 4 VO) with molecular weights ranging from 499 to 863 were the second most abundant. In addition, other compound types having the formula C n H 2n+z N 4 VO, where z ranges from -32 to -50, excluding -46, were found. Additional saturated and aromatic rings present in compounds with more negative z numbers would appreciably alter their UV-visible absorption spectra relative to those of etioporphyrins. That is, the response of the other porphyrin types would be lower than that of etioporphyrins at 570 nm, the wavelength used for the determination of vanadyl porphyrins. Thus, the broad distribution of vanadyl porphyrin types identified in the fraction by low-eV HR/MS strongly suggests that at least a part of the apparent "nonporphyrinic vanadium" thought to be present in the fraction is, in fact, porphyrinic and is explained by the use of molar absorptivities based solely on those of etioporphyrins to calculate total porphyrin content.