The reactions of the anion radicals of p-fluoranil (p-FA), p-chloranit (p-CA) and p-bromanil (p-BA) with Mg2+ or Ba2+ (M2+) were analyzed in homogeneous acetonitrile (AN) solution as well as on a platinum electrode surface. Using a stopped-flow method, the anion radicals of p-FA, p-CA and p-BA (p-XA(.-)) could be generated through the electron transfer between p-XA and decamethylferrocene (DMFc), so that the reactions with M2+ could be followed easily in homogeneous solution. While the reactivity of p-XA(.-) toward Mg2+ was almost identical, the order of the reactivity was clearly determined to be p-CA(.-) > p-BA(.-) > p-FA(.-) in the reactions with Ba2+. Correlated with this reactivity, the prior formation of Ba2+(p-CA(.-))(2) on the electrode surface was confirmed in the observation of the competitive electrochemical reactions. In particular, the transformation from Ba2+(p-BA(.-))(2) into Ba2+(p-CA(.-))(2) could be observed at the slower scan rate when the concentration of Ba2+ was the same as the total amount of p-FA, p-CA and p-BA in AN solution.