Three-phase hydrate equilibria of the methane + water + cyclic ether and nitrogen + water + cyclic ether systems were measured by employing the temperature search method. Three cyclic ethers of THF, propylene oxide and 1,4-dioxane were chosen and their concentration was fixed at 3 mol% relative to water. The addition of cyclic ethers caused the hydrate equilibrium pressure to be drastically lowered at a specified temperature and equivalently the hydrate equilibrium temperature to be greatly raised at a specified pressure. This stabilization effect on hydrate formation was found to be the highest for THF solution and in the order of THF > propylene oxide > 1,4-dioxane > acetone. A new thermodynamic model was developed to describe the phase equilibria of mixed hydrates containing two guest molecules. The Soave-Redlich-Kwong equation of state (SRK-EOS) was used with the modified Huron-Vidal (MHV2) mixing rule incorporated with the modified UNIFAC. Both the predicted and experimental equilibrium data agreed well over whole temperature and pressure ranges investigated in this study.