The electrochemical reduction at platinum electrodes of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the presence of Na+ in acetonitrile solution was studied using various electrochemical measurements under conditions with significant mass transport, namely channel flow- and sono-voltammetries. In channel electrode voltammetry, the presence of Na+ was observed to change the voltammetric response from that of pure DDQ alone. In particular, at potentials corresponding, in the absence of Na+, to the two electron reduction step of DDQ, a spike-like increase and decrease of reductive current was observed on negative going potential scans after which the current returned to the level expected for a one-electron reduction. This behavior can be explained by the following reaction scheme with the formation of a passivated surface due to the adsorption of Na-2(+) DDQ(2-).DDQ(2-) + Na+ --> Na+ DDQ(2-)Na+ DDQ(2-) --> Na+ DDQ(ads)(2-) Na+ DDQ(ads)(2-) +Na+ DDQ(ads)(2-) --> Na-2(+) DDQ(ads)(2-) + DDQ(2-)This scheme was found to be consistent with independent sono-voltammetric data and with in situ electrochemical ESR measurements.