Conventional theories of electrochemical kinetics, which are strictly applicable for two-dimensional surfaces, predict that the current (I) increases indefinitely with an increase in the reaction rate constant (k(f)), limited only by diffusion. By comparison, the I-t transients behave differently on electrodes with a rough and porous surface (dimension, D-f > 2) than on electrodes with a flat surface (D-f = 2). The difference in the behavior is caused by the flux of the reactant toward the porous surface. The rate expression for the porous electrode is obtained under the mass transfer conditions of partial diffusion in terms of Df, and kf, by extending the well-known I-t behavior for the planar electrode. The expression predicts that for a porous electrode, the currents in I-t transients have higher values at higher k(f), but only when t is close to zero. At longer times, however, the currents are higher at lower k(f), where the rate is limited by partial diffusion, than at higher k(f), where the rate is limited by total diffusion. The implications of long-term behavior of current and the associated charge (Q) are discussed in the context of electrochemical reactors such as fuel cells.