The basic equations of the kinetics of electron transfer across an electrolyte/electrolyte solution interface, in which the effect of diffuse layers on both sides is taken into account on a simplified model of electric double layer, are presented. Two ideal processes, an inner-layer rate-determining process and a diffuse-layer rate-determining process, are discussed. Theoretical predictions derived from the basic equations are discussed. Some abnormal transfer coefficients can be explained by the theory.