We measured the isotopic compositions of methane (C₁) and ethane (C₂) of hydrate-bound gas and of dissolved gas in pore water retrieved from bottom sediments in Lake Baikal. Both structure I (sI:3%C ₂) and II (sII:14%C₂) gas hydrates are observed in the same sediment cores in Kukuy K-2 mud volcano. We found that C₂ δD of sI gas hydrate is larger than that of sII, whereas C₁ δ¹³C, C₁ δD and C₂ δ¹³C values are practically the same in both hydrate structures. δ¹³C Of C₁ and C₂ of hydrate-bound gas are several permil smaller than those in pore water, showing that the current pore water is not the source of gas hydrates. These findings lead to a new model where the sII gas hydrates were formed prior to the si hydrates.