The structure at air interface and water (W) interface of a hydrophobic ionic liquid (IL), trioctylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (ITOMA(+)][TFPB-]), has been studied using x-ray reflectometry. Multilayering of ions has been found at the IL/air interface, with the topmost ionic layer having lower density than the IL bulk. For the IL/W interface, x-ray reflectivity data depends on the phase-boundary potential across the IL/W interface. When the phase-boundary potential of W with respect to IL, Delta(W)(IL)phi, is + 0.20 V, TFPB- ions are accumulated at the topmost ionic layer on the IL side of the IL/W interface. On the other hand, when Delta(W)(IL)phi=-0.27 V, the accumulation of TOMA(+) ions occurs with bilayer thickness, which is probably due to local interaction between TOMA(+) ions at the topmost layer and at the second layer through interdigitation of their alkyl chains. To quantitatively analyze the x-ray reflectivity data, we construct a model of the electrical double layer (EDL) at the IL/W interface, by combining the Gouy-Chapman-Stem model on the W side and the Oldham model on the IL side. The constructed model predicts that the EDL on the IL side is within the topmost layer for the phase-boundary potentials in the present study, suggesting that the TOMA+ bilayer found at the negative potential results from the local interaction beyond the framework of the present mean-field theory. Even at the positive potential the surface charge density predicted by the EDL theory is significantly smaller than that estimated from x-ray reflectivity data, which implies that densification of the topmost ionic layer leads us to overestimate the surface charge density. (C) 2015 Elsevier B.V. All rights reserved.