Two-step oxidation or reduction of a molecule is considered for the case where removal (or addition) of the second electron is more difficult than that of the first. In such cases, the comproportionation reaction between the final product and the original reactant to give two molecules of the one-electron intermediate is favored and can occur in the diffusion-reaction layer. When the diffusion coefficients of the three species differ, the current seen at potentials where the two-electron reaction occurs will not be twice that seen at potentials where the one-electron reaction occurs, i.e. n(app) does not equal 2. For chronoamperometry, digital simulations have been employed to predict the dependence of n(app) on the relative diffusion coefficients. It has been shown that equality of the diffusion coefficients of the reactant and the one-electron product is sufficient to guarantee that n(app) will be 2. Diffusion coefficients of neutral tetracyanoquinodimethane (TCNQ), its radical anion and the dianion have been measured in acetonitrile. It is predicted that n(app) will be 1.98 for reduction of neutral TCNQ and 2.18 for oxidation of TCNQ(2-). Within experimental error, these values were found experimentally using normal pulse voltammetry. The effect of unequal diffusion coefficients was also observed in cyclic voltammetry and confirmed by simulation.