Vaporization enthalpies of a series of ten 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (ILs) [C(n) mim][NTf(2) ] with alkyl chain lengths of n=2, 3, 4, 6, 8, 10, 12, 14, 16, and 18 are determined by using a recently developed quartz crystal microbalance method. Due to the high sensitivity of the microbalance vapor studies can be extended to temperatures 60-100 K lower than those available with other methods. The results reveal a remarkably linear dependence of the vaporization enthalpies on the chain length at the reference temperature of 298 K.