The reduction of an equilibrated isomeric mixture of CpCo(1,3-C8H8) (1,3) and CpCo(1,5-C8H8) (1,5) to their radical anions proceeds by a square scheme mechanism. The cyclic voltammetric peak heights for the cathodic waves are dependent on analyte concentration. This is shown to arise from the influence of the homogeneous cross-reaction (1,3)(-) + 1,5 reversible arrow 1,3 + (1,5)(-) near the electrode surface. Theoretical voltammograms simulated by the fast quasi-explicit finite difference method are in agreement with the experimental voltammograms over a range of concentrations and scan rates. A lower limit for the rate of isomerization of [CpCo(1,5-C8H8)](-) to [CpCo(1,3-C8H8)](-) is 2 x 10(5) s(-1), 2 orders of magnitude higher than previous estimates. Theoretical investigations show that the coupling of the cross-reaction with the rapid (1,5)(-) to (1,3)(-) isomerization favors the homogeneous redox pathway. Analogies are made to electron-transfer-catalyzed reactions. This is thought to be the first voltammetric quantitation of the solution electron-transfer pathway for square schemes with the reactants initially at equilibrium.