A description of electron transfer (ET) by the incoherent sequential model was employed to elucidate the unidirectionality of the primary charge separation process in bacterial reaction centers (RC). The model assumes that the vibrational relaxation of the medium modes is sufficiently fast and that the system relaxes to thermal equilibrium after each ET step. ET was investigated for 5-sites (molecules) arranged in two branches. Beginning at molecule 1, ET can proceed in two directions with each branch composed of two molecules. Analysis shows that the model can successfully explain the asymmetry of primary electron transfer both in the wild type and several mutants of Rb capsulatus RC. In these cases the dependence of ET asymmetry on temperature was also evaluated. It was shown that in order to obtain the correct temperature dependence of ET asymmetry in the mutants, the superexchange mechanism operating in parallel with the sequential process must be used. (C) 2003 American Institute of Physics.