Ionic liquids(ILs)1-alkyl-3-methylimidazolium Acetate [Cnmim][OAc](n = 2,4)were prepared by the neutralization method and characterized by 1H NMR spectroscopy and differential scanning calorimetry(DSC).The enthalpies of vaporization for the two ILs were determined using thermogravimetrical analysis(TGA).In terms of ΔvapH,a new scaling of polarity,δμ,for ILs was put forward.According to the values of δμ,the judgment on the polarity of ILs is in good agreement with the experiments and the experience,and its values are very easy to calculate based on the enthalpy of vaporization and refractive index data.This paper reports the following:(1)Ionic liquids [C2mim][OAc] and [C4mim][OAc] were prepared by the neutralization method and characterized by 1H NMR spectroscopy and differential scanning calorimetry(DSC);(2)The enthalpy of vaporization at the average temperature,Tav,for the two ILs were determined.Using Verevkins method,the difference of heat capacity between the vapor phase and the liquid phase,ΔvapCp,of the two ILs was calculated based on the statistical thermodynamics.In terms of ΔvapCp,the values of ΔvapH(Tav)can be transformed into ΔvapH(298)= 138.9 ± 0.8 kJ·mol-1 for [C2mim][OAc] and ΔvapH(298)= 134.0 ± 1.7 kJ·mol-1 for [C4mim][OAc] at the reference temperature 298.15 K,respectively;(3)In terms of Hildebrand solubility parameter theory,a new scale of polarity,δμ(δμ 2 is the contribution of dipole moment to cohesive energy),for the ILs was put forward and values of δμ can be estimated easily based on the vaporization enthalpy and refractive index data,which are δμ =22.6 J1/2·cm-3/2 for [C2mim][OAc] and δμ = 18.1 J1/2·cm-3/2 for [C4mim][OAc],respectively.