Physico-chemical phenomena in atmospheric aerosol, geochemical, and biomembrane systems are strongly influenced by interfacial ion distributions, and the resulting interfacial electrostatic fields. Results from early surface potentiometry and from recent phase-resolved (phase-sensitive (PS-) and heterodyne-detected (HD-)) vibrational sum frequency generation (VSFG) spectroscopy measurements on aqueous salt solutions have been interpreted in terms of anions and cations being distributed within the interfacial region according to their surface propensity, forming an ionic double layer. This molecular-level picture has been consistent with the distribution (density) profiles obtained by MD simulations of aqueous interfaces of halide salts. Here we discuss PS- and HD-VSFG results revealing that for some oxyanion-and Mg2+-based salts, the interpretation of the electric field in terms of determining ion surface propensity and/or charge could be misleading. We also discuss the intricacies of counterion effects and the correlations with surface potential versus phase-resolved (PS- or HD-) VSFG measurements. (C) 2013 Elsevier B.V. All rights reserved.