In a previous paper (Tsuji et al. Energy Fuels 2005, 19, 869-876), we reported that the rate of clathrate hydrate formation from a simulated natural gas-a mixture of methane, ethane, and propane in a 90:7:3 molar ratio-in the presence of methylcyclohexane (MCH) significantly changed with the repetition of hydrate-forming operations and that its asymptotic level substantially exceeds the rate available in the absence of MCH. Here, we report new results in which continual chromatographic analyses of the gas-phase composition inside the hydrate-forming spray chamber gave insight into the mechanism by which the presence of MCH influences the rate of hydrate formation. The observed evolution of the gas-phase composition during each hydrate-forming operation indicates that the hydrate formed over the major portion of the operation is in structure II, although MCH may provide guest molecules to fit into the 5(12)6(8) cages of a structure-H hydrate. The difference in the rate of hydrate formation from operation to operation was found to be related to the difference in the initial gas-phase composition due to the preferential dissolution of ethane and propane from the gas phase into the liquid MCH phase before the inception of hydrate formation; the higher the initial concentrations of ethane and propane in the gas phase are, the higher the rate of successive hydrate formation is.