For the binary ionic liquid (IL)-water (W) two-phase systems formed by the ILs 1-methyl-3-octylimidazolium bis(trilluoromethanesullonyl)amide ([C(8)mim(+)][C(1)C(1)N(-)]) and tetrahexylammonium bis(trifluoromethanesulfonyl)amide, which have a common anion, the activities of [C(8)mim(+)][C(1)C(1)N(-)] in the water-saturated binary IL phase have been determined from the solubility of C(8)mim(+) in the W phase at different mole fractions of the IL (x(C8mimC1C1N)(IL)) at five different temperatures between (278 and 328) K. The variation of the activity coefficients of [C(8)mim(+)][C(1)C(1)N(-)] in the IL phase with x(C8mimC1C1N)(IL) is by and large in harmony with the regular solution model. However, small but systematic deviations from the model have been discerned and are likely to result from the phase transition of the binary ILs at x(C8mimC1C1N)(IL) approximate to 0.3, which causes changes in the microheterogeneity in the assemblies of the IL-constituent ions. The standard Gibbs energy for transfer of [C(8)mim(+)][C(1)C(1)N(-)] from the water-saturated IL to W (Delta(Ir)G(m)(C8mimC1C1N)(circle minus)(IL -> W)), which was evaluated from the temperature dependence of the solubility, is 30-34 kJ.mol(-1) over the temperature range studied. The standard enthalpy for transfer of [C(8)mim(+)][C(1)C(1)N(-)] from IL to W is positive and increases linearly from 4 kJ.mol(-1) at 283 K to 24 kJ.mol(-1) at 318 K.