Vapor pressure of an ionic liquid, 1-butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)an-tide, was first experimentally determined. Temperature dependence of vapor pressure ln P-sat (Pa) = (27.11-14.42) x 10(3)/T (K) and enthalpy of vaporization Delta(vap)H(487) = 120 +/- 5 kJ mol(-1) were obtained from measurements by the Knudsen method in the range of 458-517 K. (C) 2005 Elsevier B.V. All rights reserved.