A series of polyurethane/epoxy resin interpenetrating polymer networks (PU/ER IPN) were studied by positron annihilation lifetime spectroscopy (PALS). The effects of epoxy resin type and content on the free volume properties in IPN were investigated. We found that in PU/ER IPN, the free volume hole size and fractional free volume showed a negative deviation due to closer segmental chain packing through some chemical bonding between PU and epoxy resin. Direct relationship between the gas permeability and the free volume has been established based on the free volume theory. Experimental results revealed that the free volume plays an important role in determining the gas diffusion and permeability. A series of polyurethane / epoxy resin interpenetrating polymer networks (PU / ER IPN) were studied by positron annihilation lifetime spectroscopy (PALS). The effects of epoxy resin type and content on the free volume properties in IPN were investigated. / ER IPN, the free volume hole size and fractional free volume showed a negative deviation due to closer segmental chain packing through some chemical bonding between PU and epoxy resin. Direct relationship between the gas permeability and the free volume has been established based on the free volume theory. Experimental results revealed that the free volume plays an important role in determining the gas diffusion and permeability.