This article reports the pressure-temperature conditions for the four-phase methane vapor + aqueous liquid + hydrocarbon liquid + hydrate equilibrium in the methane + water + 2,2-dimethylpentane system. The pressure and temperature ranges are from (2.430 to 4.823) MPa and from (273.7 to 279.3) K, respectively. The four-phase equilibrium pressures of this system at temperatures below 278.5 K are lower than the three-phase methane vapor + aqueous liquid + hydrate equilibrium pressures in the 2,2-dimethylpentane free system, whereas the four-phase and three-phase equilibrium pressures are equal at temperatures above 278.5 K. This difference in the equilibrium pressures due to the addition of 2,2-dimethylpentane indicates the formation of a structure-H hydrate at temperatures below 278.5 K. The crystallographic structures of the hydrates formed in this system have been identified by powder X-ray diffraction measurements.