Three-phase equilibrium conditions for the CH4 (1) + C2H6 (2) + H2O (3) system in H-L-W-V equilibrium were determined to ascertain the effects of pressure, temperature, and gas-phase composition in the temperature region above the freezing point of water as well as to construct a 3D phase diagram. The obtained equilibrium temperature, pressure, and gas-phase compositions were in the range of (275 to 281) K, (0.7 to 2.7) MPa, and y(1) = (0.30 to 0.85), respectively. Along any given isotherm and isobar, the equilibrium pressure increased and the equilibrium temperature decreased, respectively, with increasing mole fraction of methane in the gas phase for both structure I and structure II hydrates. At constant gas-phase compositions, the system followed the exponential trend seen for pure gases, with equilibrium pressures close to that of simple ethane hydrates even at high concentrations of methane in the system. A 3D representation of the phase diagram was constructed of the system. The diagram consists of two planes due to the presence of both structure I and structure II. The structure change is seen by the intersection of the two planes, and there is no significant discontinuity in the phase plane diagram.