The density and surface tension of the ionic liquids 1-(2-aminoethyl)-3- methylimidazolium tetrafluoroborate, H2N-C2mimBF 4, and 1-(3-aminopropyl)-3-methylimidazolium tetrafluoroborate, H2N-C3mimBF4, were measured from (293.15 to 343.15) K. The coefficient of thermal expansion, molecular volume, standard molar entropy, and lattice energy were calculated using experimental density data. Meanwhile, the surface entropy and enthalpy were calculated from surface tension data. The critical temperature of the ionic liquids was estimated using the Guggenheim and Eötvos equations. The values were then used to estimate the boiling temperature of the ionic liquids according to methods of Rebelo et al. The molar enthalpy vaporization, �lgHm0, of the ionic liquid at 298.15 K was estimated using the methods of Kabo et al. According to the interstice model, the thermal expansion coefficient of the ionic liquids, α, was calculated, and the result was in very good agreement with the experimental value. © 2011 American Chemical Society.