Structure of ionic liquid (IL) at the electrified IL vertical bar Hg interface has been studied using in situ spectroscopic ellipsometry (SE) for four ILs. A model incorporating two layers at the interface, one for the surface charge layer at the Hg side and the other for an ionic layer at the IL side, well reproduces the SE results. The model analysis suggests that the density of the ionic layer becomes high at more positive potentials for all the four ILs. Tendency for the increase in the density depends on the ILs and is likely to reflect structure and chemical nature of IL-constituting ions. The influence of the Hg surface charge on the SE data is negligible in the potential region studied. (C) 2013 Elsevier B.V. All rights reserved.