Theory is developed to account for the variation of the voltammetric half-wave potential with the rate of mass transport at a channel electrode for the case where an electrochemically reversible electron transfer is followed by the (partially) chemically reversible dimerisation of the electrode product. A working surface is presented which allows the analysis of experimental data and it is shown that when the homogeneous dimerisation kinetics are sufficiently rapid the half-wave potential becomes independent of the rate of mass transport but is shifted from the true formal potential. Experiments are reported on the reduction of the methyl viologen di-cation in aqueous solution. The resulting mono-cation is believed to undergo a reversible dimerisation forming a pi-dimer, and the possibility of further aggregation is investigated. Measurements of the half-wave potential as a function of solution flow-rate and concentration are consistent with dimer formation and permit the inference of a value of -0.703 +/- 0.003V (vs. SCE) for the one-electron reduction of the di-cation.