Tetra-n-butyl ammonium bromide (TBAB) forms a semi-clathrate hydrate crystal with water molecules even under atmospheric pressure. We found that TBAB hydrate could encage methane molecules in mixtures of methane and propane or methane and ethane. Our preliminary result of single-crystal analysis using X-ray shows that TBAB hydrate has empty cages, all of which are small, dodecahedral cages. Therefore, TBAB hydrate crystals can be used to separate small gas molecules which fit in these dodecahedral cages. We concluded that these empty unisized cages of TBAB hydrate crystals function as a sieve for gas molecules.