The isothermal phase equilibria of the carbon dioxide + cyclopropane mixed-gas hydrate system were investigated by means of static temperature measurement and Raman spectroscopic analysis. Raman spectra indicated that the crystal Structure of the carbon dioxide + cyclopropane mixed-gas hydrate changes from structure-I to structure-II and back to structure-I with an increase of the equilibrium carbon dioxide composition at 279.15 K, while each simple gas hydrate belongs to structure-I at the temperature. Whereas, unlike 279.15 K, no structural phase transition Occurs along the isothermal stability boundary at 284.15 K. (C) 2009 Elsevier B.V. All rights reserved.