The kinetics of gas hydrate growth from binary CH4/C 2H6 and CH4/C3H8 and ternary CH4/C2H6/C3H8 gas mixtures were obtained by the gas uptake method in a semibatch stirred vessel at constant pressure and a temperature of 273.7 K. These data are of interest for the design of facilities for natural gas storage and transportation in the solid (hydrate) state. During hydrate formation, samples from the gas phase were taken and analyzed by gas chromatography. It was found that the molar composition of CH4 in the vapor phase increased as hydrate crystallization progressed. The observed fractionation effect (enrichment of the hydrate phase with propane) complicates the natural gas storage process. The fractionation effect was also confirmed with molecular-level studies where hydrate from the CH4/C2H6/C3H 8 gas mixture was characterized by powder X-ray diffraction (PXRD), NMR, and Raman spectroscopy. The hydrate phase composition and cage occupancy of each gas were calculated with the help of information obtained by Raman spectroscopy, gas chromatography, and PXRD. The results were consistent with those obtained by NMR. The composition of the gas phase and the hydrate are found to evolve over time, suggesting that kinetic and transport factors contribute in addition to thermodynamics. © 2008 American Institute of Chemical Engineers.