With the use of isothermogravimetrical analysis, the enthalpies of vaporization, Delta H-g(1)l(m)degrees (T-av), at the average temperature, T-av = 445.65 K, for the ionic liquids (ILs) 1-alkyl-3-methylimidazolium propionate [C(n)mim][Pro](n = 4, 5, 6) were determined. Using Verevkins method, the difference of heat capacities between the vapor phase and the liquid phase, Delta C-g(1)l(p)degrees(m), for [C(n)mim][Pro](n = 2, 3, 4, 5, 6), were calculated based on the statistical thermodynamics. Therefore, with the use of Delta C-g(1)l(p)degrees(m), the values of Delta H-g(1)l(m)degrees (T-av) were transformed into Delta H-g(1)l(m)degrees(298), 126.8, 130.3, and 136.5 for [C(n)mim][Pro](n = 4, 5, 6), respectively. In terms of the new scale of polarity for ILs, the order of the polarity of [C(n)mim][Pro](n = 2, 3, 4, 5, 6) was predicted, that is, the polarity decreases with increasing methylene. A new model of the relationship between the surface tension and the enthalpy of vaporization for aprotic ILs was put forward and used to predict the surface tension for [C(n)mim][Pro](n = 2, 3, 4, 5, 6) and others. The predicted surface tension for the ILs is in good agreement with the experimental one.