The transition flux formula for the coupling matrix element of long-distance electron transfer reactions is discussed. Here we present a new derivation which is based on the Golden Rule approach. The electronit Franck-Condon factor that appears in the multielectronic formulation of the coupling element is discussed using the concept of tunneling, time. An application of the tunneling flux theory to electron transfer reactions in a model system based on the low-potential heme and high-potential heme (herne b(L))/(heme b(H)) redox pair of ubiquinol:cytothrome c oxidoreductase complex, is described; the results are compared to those obtained by measuring energy splitting of the donor/acceptor multielectronic states and the direct calculation method.