Abstract: In this study, trihexyltetradecylphosphonium octylsulfosuccinate [P_6,6,6,14][docusate] was synthesised by anion metathesis using sodium octylsulfosuccinate salt. The molecular structure of the synthesised IL was confirmed by using ¹H NMR and elemental analysis and also physical properties such as density, viscosity and refractive index were studied as a function of temperature at the atmospheric pressure. The experimental values of density and refractive index decrease linearly with increasing temperature. The density, refractive index and viscosity of the present IL at 298.15 K are 0.9631 g cm^-3, 1.47190 and 1806.1 mPa.s respectively. The results show that this IL possesses higher viscosity, similar density and refractive index compared to the other trihexyltetradecylphosphonium ILs.