The equilibrium conditions of methane hydrate in the presence of additives of cyclopentanone (C5H8O) or 4-hydroxy-4-methyl-2-pentanone (C6H12O2, also known as diacetone alcohol) in aqueous solutions were experimentally measured in this study. The hydrate-liquid water-vapor (H-L-w-V) three-phase equilibrium conditions were determined by employing the isochoric method within a high pressure cell at low temperature conditions. New experimental results are reported for pressures ranging from 7 to 13 MPa with various concentrations of these additives. It is demonstrated that the cyclopentanone additive has a promotion effect on the formation of methane hydrate, while the other additive of 4-hydroxy-4-methyl-2-pentanone shows an inhibition effect. The structures of methane hydrates with each of the two additives were determined using the Clausius-Clapeyron equation. To simulate the salinity of the seawater environment, this study also measured the hydrate-liquid water-vapor (H-L-w-V) three-phase equilibrium temperatures and pressures by adding both additives to brine solution consisted of NaCl at 0.035 mass fraction. In both cases, the additional component of NaCl in the aqueous phase reduces the equilibrium temperature by about 2 K. (C) 2014 Elsevier B.V. All rights reserved.