Two propionic acid ionic liquids (PrAILs) [C(n)mim][Pro] (n = 2, 3) (1-alkyl-3-methylimidazolium propionate) were prepared by the neutralisation method and characterised by H-1 NMR spectroscopy and differential scanning calorimetry (DSC). The values of vaporisation enthalpies, Delta H-vap(m)o, for two PrAILs [C(n)mim][Pro] (n = 2, 3) were determined using a simple, convenient, and highly reproducible thermogravimetric approach. The enthalpy of vaporisation, Delta H-vap(m)o, for ionic liquids (ILs) is assumed to be composed of two parts: one part corresponds with the induced energy, Delta H-vap(m)o (n), and the other part corresponds with orientation energy from permanent dipole moment of ion pair in ILs, DvapHo mdl_. Using the values of Delta H-vap(m)o, Delta H-vap(m)o(n) and Delta H-vap(m)o(mu), cohesive energy density, delta(2) (delta is the solubility parameter), the contribution of inducted energy, delta(2)(n), and the contribution of orientation energy, delta(2)(mu), were obtained. If a liquid only has delta(n) then it is a non-polar liquid, and if a liquid not only has delta(n) also has delta(mu) then it is a polar liquid. Since the ion pairs in ILs have permanent dipole moment, ionic liquid has a certain polarity. Therefore, using delta(mu) as the polarity scaling of ILs is very convenient because the values of delta(mu) are very easy to calculate from the enthalpy of vaporisation and refractive index data. (C) 2013 Elsevier Ltd. All rights reserved.