The dissociation point is the condition where three phases of hydrate (H), liquid water (L-w), and vapor (V) coexist. This is the thermodynamic phase equilibrium property and is essential to the exploration and utilization of new energy sources of methane hydrates. Additive compounds are usually used in methane hydrate research. The measurements of the dissociation conditions or the three-phase (H-L-w-V) equilibrium temperatures and pressures are required for ternary mixtures with additives. The dissociation points of the methane hydrate mixture in the presence of each additive of (0.10 to 0.30 mass fraction) acetamide, (0.05 and 0.08 mass fraction) cyclopentanol, and (0.10 and 0.20 mass fraction) 1,3-dioxane in aqueous solutions were experimentally measured in this study. To simulate the seawater condition, the dissociation points of these additives in brine solution were also investigated. The isochoric method with an apparatus operated at high-pressure and low-temperature conditions was applied. New experimental data are reported within the pressure range of 7 to 13 MPa for methane hydrate mixtures. It is demonstrated that the addition of acetamide yielded an inhibition effect on the formation of the methane hydrate mixture. The most inhibition result was 8.7 K for an isobaric state. On the other hand, the additives of cyclopentanol and 1,3-dioxane behaved as promotors. The largest promotion results of these two additive compounds at specific isobaric conditions were 6.8 and 13.7 K, respectively.