Experimental permeability and diffusivity values for CO2 and CH4 through imidazolium-based ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([hmim][Tf2N]) were determined in the temperature range of 300-320 K using temperature correction factor defined in our previous study. According to literature, experimental values of permeability and diffusivity obtained in this study for CO2 in [hmim][Tf2N], showed good agreement with predictive models reported by other researchers. In addition, experimental values of permeability and diffusivity for CH4 in [hmim][Tf2N] as a function of pressure have been reported in this study. Considering the results of present study and those of our previous study on another imidazolium-based ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), predictive models for CH4 permeability and diffusivity through this group of ionic liquids were developed. (C) 2011 Elsevier B.V. All rights reserved.