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The reaction of heme proteins and H2O2 is thought to proceed via two-electron oxidation, resulting in the formation of ferryl heme (Fe^IV&dbond;O), and a protein radical. The ferryl-radical form of heme proteins would then catalyze the oxidation of molecules. Incubation of heme proteins with H2O2 in the presence of methyl-nitrosopropane (MNP) resulted in the detection of electron paramagnetic resonance (EPR) spectra characteristic of a highly immobilized nitroxide. Lineshapes of the EPR spectra were consistent with a slow tumbling molecule as it is evident from the broadening of the high field lines. This is consistent with the high molecular mass of the radical adduct. The goal of this work is, using a computer program based on the stochastic Liouville equation, to simulate directly these slow-motional EPR spectra and obtain information on magnetic and dynamic parameters of the complexes. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 249–254, 1999