Vapor compositions of methane and propane mixed gas in a batch-type reactor were measured by gas chromatography during hydrate crystallization at 274 K with molar ratios of propane below 10 vol %. The volume ratio of propane in the vapor decreased as the hydrate crystallization progressed. When the initial propane concentration was between 4 and 8 vol %, rapid gas consumption occurred for about I h, causing an initial pressure drop, and after a temporary stabilization of the pressure, a second pressure drop occurred; that is, hydrate crystallization occurred in two steps. X-Ray diffraction and Raman spectroscopic analyses on samples taken from the reactor during each step revealed that the structure II methane-propane mixed gas hydrates crystallized in the first step, and structure I methane hydrates in the second step. This process was observed only when the partial pressure of methane was above the equilibrium pressure of methane hydrate at the end of the first step. (C) 2004 American Institute of Chemical Engineers.