Isotopic fractionation of carbon and hydrogen in methane and ethane during the formation of gas hydrates was investigated. The gas hydrate samples were experimentally prepared in a pressure cell and isotopic compositions of both residual and hydrate-bound gases were measured. δD of hydrate-bound molecules of methane and ethane hydrates was several per mil lower than that of residual gas molecules in the formation processes, while there was no difference in the case of δ¹³C. These isotopic differences in δD are enough small for discussing the source types of hydrate-bound gases using the δ¹³C-δD diagram of Whiticar et al. [1986]. These results may provide useful insight into the formation process of gas hydrates.