In the first part of this paper the electrochemical transfer of alkali cations (M+) assisted by monensin (HX) across the water \ 1,2-dichloroethane (DCE) interface at pH < 5, combined with a chemical exchange reaction at 5 < pH < 9, is proposed as the only mechanism responsible for the transfer of these cations. At pH > 9 the current is voltammetrically negligible. An equation for the dependence of Delta (w)(o)phi (1/2) on pH and Na+ concentration is developed. In the second part of the gaper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me-(w)(2+) + HX(o) reversible arrow MeHX(o)2+) and (Me-(w)(2+) + X-(o)(-) reversible arrow MeX(o)+) are responsible for the peaks observed at pH < 5.0 and at pH > 9.0, respectively. As expected, in both cases DeltaE(p) = 0.030 V while at intermediate pH the electrochemical exchange reaction (Me-(w)(2+) + HX(o) reversible arrow MeX(o)+ + H-(w)(+)) is proposed. The net charge transfer of + 1 at the interface accounts for DeltaE(p) = 0.060 V for the peak observed DeltaE(p) in agreement with the hyper-Nernstian slope of 60 mV found for Ca2+ and Ba2+ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba2+. and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.