A @F02321@ whose magnitude is controlled by the rate at which a reactant in an electrochemical process diffuses toward an electrode-solution @I03082@ (and, sometimes, by the rate at which a product diffuses away from that @I03082@). For the @M03804@ there are two common situations in which a @D01716@ current can be observed. In one, the rate of formation of B from electroinactive C is small and the current is governed by the rate of @D01716@ of B toward the electrode surface. In the other, C predominates at equilibrium in the bulk of the solution, but its @T06446@ into B is fast; C diffuses to the vicinity of the electrode surface and is there rapidly converted into B, which is reduced.