A series of 1-(2-alkoxyethyl)-3-methylimidazolium + [anion] ionic liquids including 1-(2-methoxyethyl) -3-methylimidazolium chloride, 1-(2-methoxyethyl) -3-methyl-imidazolium DL-threonine, and 1-(2-ethoxyethyl) -3-methylimidazolium DL-threonine were synthesized and characterized to determine how initial decomposition temperature, vaporization enthalpy, and thermal stability were affected by changes in alkoxy chain length and anion. An isothermal thermogravimetric experiment was carried out on a series of ionic liquids (ILs) by using a thermogravimetric analyzer. Combining with the Langmuir equation and Clausius-Clapeyron equation, experimental data were used to calculate vaporization enthalpy at an average temperature. Comparing the values of vaporization enthalpy for three ILs, the order is [EOEMIM]Thr > [MOEMIM]Cl > [MOEMIM]Thr. Thus, the influence of a changing alkoxy chain on vaporization enthalpy of ILs is large.