The ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii requires adenosylcobalamin (AdoCbl) as a cofactor to catalyze the conversion of nucleotides to deoxynucleotides. RTPR has previously been shown to catalyze the homolytic cleavage of the carbon-cobalt bond of AdoCbl, and the resulting paramagnetic species has been characterized by rapid freeze-quench EPR spectroscopy (Orme-Johnson, W. Fl.; Beinert, H.; Blakley, R. L. J. Biol. Chem. 1974, 249, 2338-2343. Licht, S.; Gerfen, G. J.; Stubbe, J. Science 1996, 271, 477-481). This study presents simulations of X- and Q-band EPR spectra of this intermediate. Modeling this species as a thiyl radical coupled to cob(II)alamin by electron-electron exchange and dipolar interactions yields reasonable fits to spectra obtained at both microwave frequencies, whereas simulations that employ a single-spin model do not. This modeling provides support for the intermediacy of a thiyl radical in this system. The techniques employed here may prove generally useful in simulation of similar spectra observed in other B-12-dependent enzyme systems.