P-T-X phase diagrams of the ternary tetrabutylammonium bromide (TBAB)-methane-water-systems have been studied by differential thermal analysis under pressure up to 40 MPa and 1.2, 2.6, and 4.1 mol % TBAB. It was shown by X-ray powder diffraction that a double hydrate of TBAB + methane of hexagonal structure I (HS-I) was formed in the TBAB-methane-water system under the studied conditions. The methane capacity of the double hydrates has been measured by volumetric analysis. It was concluded that substitutional solid solution with any significant displacement of the TBA cation by methane molecules was not formed.