The structure of gas hydrates is of the clathrate type coresponding to a re-organisation of the water molecules around gas molecules. The cages consist of polyhedra of water molecules formed by hydrogen bonds and stabilized by the encapsulated gas molecule. One considers that the ecapsulated molecule remains at the gas state into a solid structure consisting of a 3D organization of different polyhedra cavities.
 

Five of these cages can be involved in gas hydrates constitution, resulting in the three identified gas hydrates structures, sI, sII and sH. For example, methane hydrate is a structure I cubic crystal, which results from the combination of two small pentagonal dodecahedron (called 512 into the Jeffrey's definition) and six tetrakaidecahedron (51262) cages, so that its stoichiometric unit cell formula is 8 CH4, 46 H2O. The structure SI is cubic  In the figure are represented different hydrocarbon molecules which can occupy the centre of the two cavities involved in structure sI. The cavity 512 is the smaller one (average radius of 0.39 nm) and can be stabilized only by small molecules such as methane, propane and cyclopropane. The cavity 51262 is bigger (average radius of 0.43 nm) and molecules as propane, i-butane and n-butane can enter into it. The cavities are represented here only in a simplified form. One can observe the polyhedra's edge and atoms of oxygens which are localised on the corners. Atoms of hydrogen are not represented but they are on the middle of the edges. The angle between two edges is 109,4° which is a value close to the natural angle in water molecule. The distance between two oxygens is 0.276 nm.