We describe a new, rapid method for determining the concentration of vanadyl porphyrins (VO2+-P) associated with the kerogen of bituminous sedimentary rocks using electron spin resonance (ESR). The method is simple, straightforward and inexpensive. Several concentrations of a vanadyl (VO2+) standard dissolved in glycerol-lignite mixture were prepared. The VO2+ concentrations ranged from 100 to 1000 ppm. The anisotropic ESR spectra of both the standards and kerogen samples were recorded at room temperature and the integrated areas of the pre-selected ESR line (attributed to nuclear spin m(t) = -5/2) were computed. The concentrations of VO2+ found in the kerogen samples were calculated using the relative ratio of the integrated areas for the standards and the kerogen samples. The VO2+-P concentrations of the kerogen materials were then calculated using 350 as the mean molecular weight of these species. Quantitative determination of VO2+-P in the kerogen fractions in the range of 800-8000 ppm and higher is feasible by the method reported. The method of analysis was also extended to the asphaltene samples (enriched with VO2+-P) and a coal sample containing non-porphyrin VO2+ associated with its organic fraction. (C) 2000 Elsevier Science Ltd. All rights reserved.