Among a variety of cyclic ether, cyclic ester, and cyclic ketone compounds, six new formers were found to form binary sII or sH hydrates with CH4 gas. Hydrate-phase equilibria for all the hydrate formers were measured. The results obtained showed distinct relationships between the hydrate-phase equilibrium curve and the molecular size of the guests. In addition, 2-methyltetrahydrofuran and 3-methyltetrahydrofuran, or 4-methyl-1,3-dioxane and 4-methyl-1,3-dioxolane, showed different hydrate structures even though they have similar chemical structures. Such structural differences can provide useful information on the critical guest size, which determines hydrate crystal structures according to the size of the captured guest.