A model including an isotopic exchange reaction of T+, HT, H2 and H+ dissolved in Flibe is proposed to explain the tritium release behavior from Flibe in an in-pile experiment. The temporal change of HT release rate for about two hours after the start of tritium generation observed by the experiment was well reproduced by the model. The steady-state values of HT concentration in Flibe in case of low H2 partial pressure calculated from the experimental data are limited by the concentration of molecular hydrogen (H2+HT) in Flibe. H2 supply from the gas phase is a rate-determining mechanism for the isotopic exchange reaction.