The electrochemical transfer of alkaline-earth cations (Me2+) assisted by monensin (HX) across the water \ 1.2-dichloroethane (DCE) in the presence of alkali cations (M+) is reported. A shift in the peak potential for Ba2+ transfer and an increase in DeltaE(rho) from 0.030 to 0.060 V are the principal effects produced by the presence of M+. These findings depend on pH and the relative concentrations of both cations. The following chemical exchange reaction M-(w)(+) + HX(c) + H2O reversible arrow MX(o) + H3O+ coupled to the electrochemical transfer of Ba2+ accounts for AEP = 0.060 V of the peak observed as a consequence of the net charge transfer of + I at the interface. These results are in agreement with the hyper-Nernstian slope of 60 mV found for Ca2+ and Ba2+ ion selective electrodes based on antibiotics with carboxylic groups at intermediate pH values. A theoretical equation for the dependence of Delta(o)(w)phi(1 2) for Me2+ with pH and M+ activity was developed.