The Nankai hydrate field, Japan, is an example of gas-hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, 129I/I ratios together with halogen concentrations were measured in a set of pore-water samples collected from two boreholes in the Nankai hydrate field. Iodine concentrations are between 100 and 230 μM, i.e., strongly enriched compared to seawater, while Cl concentrations were found to be close to that of seawater. Except for one sample, 129I/I ratios are between 180 and 520 × 10−15, giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments (<2 Ma) and subducting marine sediments (<21 Ma) in this area, iodine (and, by association, methane in the gas hydrates) must have been derived from source formations located in the continental side of the subduction zone. The results do not support derivation of gas hydrates from present host sediments or currently subducting sediments, but could be related to release and long-time recycling of fluids from marine formations of early Tertiary age.