The clathrate hydrate formation from a model natural gas, i.e., a mixture of methane, ethane, and propane in a 90:7:3 molar ratio, under a constant pressure was experimentally investigated, focusing on the compositional evolution of hydrate crystals formed inside a gas-bubbling-type reactor during each semibatch hydrate-forming operation. The experimental system used in this study was specially designed for obtaining several hydrate samples formed at different, arbitrarily selected stages during each hydrate-forming operation. Each hydrate sample was analyzed by a gas-chromatograph to determine the mole fractions of methane, ethane and propane encaged in the hydrate. These analyses revealed a monotonic increase in the methane fraction and decreases in the ethane and propane fractions during each operation until a quasi-steady state was established. Powder X-ray diffraction analyses showed that both structure-I and structure-II crystals were simultaneously formed during the quasi-steady period. The compositional evolution of the hydrates formed during the early stages before the quasi-steady state was reached deviated from corresponding predictions based on the thermodynamic-simulation scheme previously reported. A hypothetical explanation for the discrepancy between the experimental and simulation-based results was provided. (C) 2013 Elsevier Ltd. All rights reserved.