We succeeded in formation of the stable HS-I hydrate with tri-n-butyl, n-hexylammonium chloride (N4446Cl, where N, 4 and 6 denote the central nitrogen, n-butyl chain and n-hexyl chain, respectively) under gas pressures of both methane and carbon dioxide and also atmospheric pressure. The single crystals of the N4446Cl + CH4 hydrates are shown in Fig. in 5 Supporting Information. The present structure was determined to be hexagonal with 𝑃𝑃6 � m2 space group. The lattice size was a = 12.170 Å and c = 12.496 Å, which is close to that estimated by Jeffrey, a = 12.5 and c = 12.5 Å . Fig. shows a whole structure of the present HS-I hydrate. The prototype of HS-I structure is a hexagonal structure which consists of dodecahedron (D or 5 12 ), tetrakaidecahedron (T or 5 12 6 2 ), and pentakaidecahedron (P or 5 12 6 3 ) where 5 and 6 denotes 10 pentagonal and hexagonal faces, respectively . Its cage composition is 3D + 2T + 2P, where superscript denotes number of faces in polyhedron. Although the present cages have many disorders, they can be ordered by choosing symmetric positions for their vertices. Fig. shows the symmetries of the T and P cages in the ideally ordered forms. They are 6mm and 6 � m2, respectively, and identical with the predicted true forms . The whole cage combination of the unit cell is 6D•2T 4 •2T 2 P 2 . In pairs of T cages (T 2 ), the butyl chains settled like an anchor, allowing the rest of the two arms to be positioned in any two of the six P cages that surround these T cages. The other butyl and hexyl chains occupied the P cages. The nitrogen atom of the cation locates at the junction of T 2 P 2 cage, however, in the present structure it can occupy three of six vertices of hexagonal face where two T cages coupled. These vertices are strongly 20 disordered points where nitrogen of the cation and chlorine anion appeared alternately. The long hexyl chain fits into the P cage and it may stabilize the P cage in a natural form compared to the P cage in the orthorhombic structure. Fig. ) shows a view of the structure from [1, 1, 0] plane. The T cage aligned toward the c axis like pillar of the structure. On the a-b plane, the D cage and the P cage alternately layered. The disordered P cage has a symmetry of m. The symmetry of the ideally ordered P cage was 6 � m2 which was identical with that originally predicted by Jeffrey and that found in Br2 true clathrate hydrate . Contrary, the cage symmetry of the P cage found in the conventional orthorhombic Pmma (34) and Imma structure was m which is quite lower than the ideally ordered one. In the ordered P cage as shown in Fig. (c), imaginary Cl-Cl bonds appeared three times at an equivalent position, where simultaneous occupancy of both Cl sites cannot actually happen. However, X-ray crystallography allows to observe the HS-I structure with this symmetric P cage using site occupancy by averaging data over time and space. It is noteworthy that the anion may play an important role in forming the ordered P cage by supporting with longer Cl-O bond distances than O-O which cannot be realized only by water molecules. The T cage has also strong disorders in its lattice. In the ordered form, one of its two hexagonal faces only consists of chlorine, and these chlorine ions are identical with that formed ordered P cage, i.e., sites labeled as Cl2 and Cl06 in the provided CIF file (See Supplementary Materials). Differing from the conventional semiclathrate hydrate structures (34,35), only one type of D cage is found in the present HS-I structure.