In this account, surface tensions of mercury/ethylammonium nitrate (EAN) aqueous solutions at different compositions as a function of the potential have been measured. Using the first and the second derivative of surface tension curves, surface charge and differential capacitance have been obtained. Assumptions based on simple hypotheses and comparison of the EAN behavior with respect to ammonium nitrate (AN) provided information about the ionic structure at the interface mercury/EAN solutions. The results from the electrocapillarity maximum data suggest excess adsorption of the nitrate anions at low EAN concentrations and excess adsorption of the ethylammonium cations at higher concentrations. The results from the differential capacitance indicate that the ethyl group of the EAN cation would preferably be oriented towards the metal mercury. (C) 2011 Elsevier Ltd. All rights reserved.