Biphasic systems involving ionic liquids (ILs) form the basis of many applications in extractions and reactions In this work, the liquid liquid phase equilibrium, density. molar volume, excess molar volume, and interfacial and surface tensions are measured for saturated and subsaturated mixtures of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf(2)N)] and 1-octene at four different temperatures, (10, 25, 50, and 75) degrees C. Many of the propel ties of the IL phase change significantly with the increasing concentration of I-octene The norandom two-liquid (NRTL) activity coefficient model was utilized to model the mutual solubility of two liquids 1-Octene is fairly soluble in the [HMIm][Tf(2)N], but the IL solubility in the I-octene is very small The excess molar volume for the mixture of ILs and I-octene is slightly negative for all isotherms and becomes mole negative with an increase in temperature The air liquid surface tension decreases with increasing 1-octene concentration, and the saturated [HMIm][Tf(2)N] + I-octene interfacial tension decreases significantly at higher temperatures.