The 2:1 mixture of LiF and BeF2 (Flibe) is a potential liquid tritium breeding material for fusion reactors, because of low electric conductivity, high chemical stability in air, etc. We have been studying tritium release behavior from Flibe by in-pile tritium release experiment in the fast neutron source reactor “YAYOI” of the University of Tokyo. About 100g of Flibe was utilized for neutron irradiation to produce tritium, and the tritium released from the specimen was swept by making purge gas flowing over the specimen. Tritium release rate increased with elapsed time after the start of irradiation and a steady state was attained in about four hours at 873K in case of H2 purge gas. Released chemical forms of tritium were HT and TF, and their proportions depended not on the kind of container and tubing materials, but on the chemical composition of purge gas and the dehumidification time of specimen at high temperatures. Tritium generated as T+ in Flibe was released by way of two routes; (1) direct release to purge gas as TF and (2) release to purge gas after converted to HT by the isotopic exchange reaction with H2. The reaction rate was controlled by the H2 concentration and F+ potential in the system. In case of high hydrogen concentration and low F+ potential in the system, tritium was released as HT with a relatively high rate. In case of high F+ potential in the system, on the other hand, tritium was released as TF with a low rate.