The present study experimentally demonstrated clathrate hydrate formation in the systems of (methane + water + each of the three methylcyclohexanone isomers, i.e., 2-methylcyclohexanone, 3-methylcyclohexanone, and 4-methylcyclohexanone) and measured the first data of the quadruple (water rich liquid + hydrate + methylcyclohexanone rich liquid + methane rich vapor) equilibrium pressure and temperature conditions in these systems over the temperatures from T = 273 K to T = 281 K. In the three systems with methylcyclohexanone, the measured equilibrium pressure at each given temperature is similar to1.3 MPa lower than that in a structure-I hydrate forming (methane + water) system without any methylcyclohexanone, which suggests the formation of structure-H hydrates with methylcyclohexanones as large-molecule guest substances. Among the three systems, 3-methyleyclohexanone provides the highest equilibrium pressure, and 2-methylcyclohexanone, the lowest. (C) 2003 Elsevier Ltd. All rights reserved.