The rate equations for a two-electron reduction, also involving heterogeneous protonation steps, in which several pathways are followed in parallel have been derived using two procedures. The first is based on the "scheme of squares" to which the kinetic laws are applied. The second is a more intuitive method formulated in terms of equivalent resistances which allows the possible mechanisms to be studied easily in a more systematic way. The charge transfer resistance versus potential curves for pyrimidine reduction in the pH range 6.5-7.7 are analysed in terms of the rate equations deduced for several branched mechanisms. The influence of the potential and pH allows the pathways preferred by the system to be determined.