A binary room-temperature ionic liquid (RTIL) composed of bis(pentafluoroethanesulfonyl)amide (C(2)C(2)N(-)) salts of tetraheptylammonium (THpA(+)) and N-tetradecylisoquinolinium (C(14)Iq(+)) undergoes a phase transition upon increasing the mole fraction of C(14)Iq(+) (x) in the bulk RTIL. The initial decrease with x of the interfacial tension (gamma) at the interface between water (W) and the binary RTIL reaches a break point at x approximately 0.2 irrespective of the values of the phase-boundary potential. The surface tension at RTIL|air interface and the conductivity of the binary RTIL support that the break point at x = 0.2 at the RTIL|W interface is attributable to the change of the bulk property. However, unlike the micelle formation of a surfactant solution, a further increase in x gives rise to a further change in gamma. Whereas the phase transition at x = 0.2 does not depend on the applied potential (E) across the RTIL|W interface, the mode of the change in gamma at x > 0.2 strongly depends on E and the apparent deficit of C(14)Iq(+) at the interface is more pronounced when E is closer to the point of zero charge.