An experimental investigation on mass transfer of CO2 through the liquid CO2-water interface is reported. At high pressures and low temperatures, water at the liquid CO2-water interface has a quasi-crystalline structure which, locally, may resemble a hydrate crystal element. If the pressure and temperature of the system fall within the hydrate-formation region, then hydrate will form rapidly at the interface; in this case, the quasi-crystal becomes a true crystal.. It is found in this study that the interfacial crystallization does not cause a dramatic decrease in the rate of mass transfer, and for both cases with and without an interfacial hydrate layer the overall mass-transfer coefficients are of order of 10(-7) m s(-1) A mechanism for the interfacial mass-transfer is proposed, which reasonably explains the behavior of the interfacial hydrate layer.