Electrocapillarity has been studied in detail at the interface between a hydrophobic ionic liquid (IL), trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, and water, where the ultraslow relaxation of the structure of the electrical double layer on the IL side of the interface exists. The response of the interfacial tension and that of the charging current to the potential step can be fitted by a double exponential model having the relaxation time constants of a few seconds and 100 s. The hysteresis in the interfacial tension in the IL diluted with nitrobenzene persists even in an almost 1 1 mixture of the IL and nitrobenzene. The thermodynamically definable double layer capacitance (C-dl) estimated from the equilibrated electrocapillary curve, that is, the interfacial tension versus potential curve, at the ILlwater interface has a maximum value of about 60 mu F cm(-2) in the vicinity of the potential of zero charge (pzc) when the aqueous phase is 0 1 mol dm(-3) LiCl. The C-dl versus the potential plot shows a characteristic camelback shape, showing a shallow minimum at the pzc, where the C-dl value is 42 mu F cm(-2). This minimum seems to reflect the contribution of C-dl on the aqueous side of the interface to the total capacitance.