The adsorption of decylsulfate (DeSO(4)(-)) and decylammonium (DeNH(3)(+)) at the 1,2-dichloroethane (DCE)vertical bar water (W) interface has been examined as a function of the phase-boundary potential by simultaneous recording of electrocapillary curves and voltammograms. The standard Gibbs energies for the adsorption of DeSO(4)(-) and DeNH(3)(+) at the DCE vertical bar W interface from the W phase depend linearly on the phase-boundary potential, having the slopes of 9.1 and -9.8 U mol(-1) V-1, respectively. These values suggest that the charged head groups of adsorbed surface-active ions are located almost outside of the diffuse part of the electrical double layer in W. It has also been demonstrated that the interface enters into the mode of the electrochemical instability in the potential region where the current is slightly augmented in comparison with that of the diffusion-limited current, that is, prior to the appearance of irregularly increased currents on the voltammogram.