Gas adsorption rates of H-2, CO2, and H-2-CO2 gas mixture (H-2/CO2=3.4) with tetra-n-butyl ammonium salt (bromide, chloride, and fluoride) semi-clathrate hydrate particles were measured at 269 K to assess their properties for gas separation. Equilibrium gas occupancies in the S-cages of the particles were in order of (high to low) for hexagonal structure-I, tetragonal structure-I, and superlattice of cubic structure-I structures with the maximum fractional occupancy by CO2 being about 40%. The CO2 diffusion rate depended on the anion size of the salt, which is attributed to distortion of the S-cage that is close to the molecular size of CO2. Simulations of semi-clathrate hydrate particles with theory showed that H-2/CO2 selectivities could be as high as 36 (3.0 mol% TBAF) and that selectivities for an ideal membrane (3.3 mol% TBAF) could be >100 (269 K, 0.3-4.5 MPa). Semi-clathrate hydrates have wide application as separation media for gas mixtures. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 992-1003, 2015