In studying the relative dissociation times of structure I (sI) and structure II (sII) gas hydrates prepared from methane + ethane mixtures, X-ray diffraction analysis indicated the simultaneous presence of both hydrate structures at conditions where only one hydrate structure was expected. The aim of this work was to develop a model describing the formation of a gas hydrate from a mixture of methane + ethane in a closed system as the hydrate forms and to validate the model by experiment. Two methods of laboratory hydrate preparation from ice particles were studied: (1) a closed-system pressure-drop formation and (2) a closed-system constant-pressure formation. Models were developed for both methods 1 and 2, and method 2 was also investigated experimentally using a mixture of mole fraction 0.897 methane to form the hydrate. Compositions of the gas were monitored periodically during the hydrate formation by gas chromatography (GC). The water-free hydrate-phase composition of methane and ethane was also measured by GC at the completion of hydrate formation. During hydrate formation, the mole fraction of methane in the gas phase increased. The model showed the same trend, although there was a discrepancy between the model and experiment that is most likely due to the assumption in the model that equilibrium between the phases exists at all times.