Experimental densities, isobaric heat capacities, and enthalpies of fusion for one sample of 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and two samples of 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide, [bmim][NTf2], are reported. Data were obtained at atmospheric pressure and, in the case of the density and the heat capacity, within the temperature intervals (283.15 to 323.15) K and (278.15 to 333.15) K for [bmim][PF6] and [bmim][NTf2], respectively. A critical analysis of the effect of impurities on the measured thermodynamic properties is performed.