The methane + cyclopropane mixed gas hydrate system has been investigated at 291.1 K by means of gas chromatography and Raman spectroscopy. Both of pure guest species generate the structure-I hydrate in the present conditions. Isothermal phase equilibria exhibit discontinuity around the equilibrium cyclopropane composition (water-free) in the gas phase of 0.20. The Raman shifts have changed bordering at the point. These results reveal that the methane + cyclopropane mixed gas hydrate generates the structure-II crystal in the methane rich region, while the structure-I crystal is generated in the cyclopropane rich region. (c) 2005 Published by Elsevier B.V.