The tritium bred in a DT fusion reactor is taken out of its blanket using helium sweep gas. The cryosorption bed using molecular sieves or activated carbon at liquid nitrogen temperature is attractive for recovery of this tritium from the view point of adsorption capacity and pressure of tritium at release. The mass transfer coefficients required to predict the breakthrough curve are discussed in this paper. The surface difiusivity included in one of them is quantitated. Its value is dependent on the adsorption site. The rate controlling step changes with the equilibrium partial pressure of the hydrogen isotope, because the mass transfer coefficient representing the intraparticle diffusion decreases with increasing equilibrium pressure. The mass transfer coefficients in desorption are estimated to be the same as those in adsorption.