The interface between explosive and binder in plastic-bonded explosives (PBXs) plays an important role in their properties such as thermal and mechanical stability, and also their performance in detonation processes. However, characterization of their interfacial micro-structures remains challenging, due to the sensitive nature of the explosive material, and the extremely thin nature of the interface. This work demonstrates a concept of characterizing interfacial structures between explosives and binders by gas permeation. The N-2 permeability data of composite films of cyclotetramethylene-tetranitramine (HMX) particles dispersed in fluororubber binder (copolymers of vinylidene fluoride and chlorotrifluoro-ethylene, F2311) were tested and fitted by using gas transport mechanism theory, e. g. the Hashemifard-Ismail-Matsuura (HIM) model, and the Knudsen diffusion equation. The results indicate the presence of voids of thickness 2.2 nm between HMX and F2311, consistent with the results of neutron reflection and thermal conductivity measurements. These interfacial voids are considered to be related to the surface roughness of HMX particles. This work provides an alternative characterization technique for, as well as a new insight into, the interface between HMX and F2311.