Microscopical Monte Carlo simulations of the electrochemical reductions of methyl fluoride and methyl chloride in water to give methyl radical and the corresponding halide anion have been performed. These are intended to be realistic models of a dissociative electron transfer reaction, a special case of an innersphere electron transfer process. Assuming a classical frame, the diabatic free energy curves corresponding to the precursor and successor complexes, in function of the reaction coordinate AE defined as the difference between the diabatic energy hypersurfaces for each configuration of the system, have been built up. The results of the computer simulations suggest that, unlike the outersphere electron transfer reactions, the Marcus’ relationship cannot be applied straightforwardly to the dissociative electron transfer reactions in solution, its validity depending on the features of the solute internal potential energy. The relation of this fact with the central limit theorem is discussed. © 1994, American Chemical Society. All rights reserved.