The utilization of the Pb-17Li eutectic as a breeding material for a D-T fusion reactor is limited by problems related to the tritium permeation from the blanket to the water-cooling system and to the environment.Experimental work is being carried out to determine the kinetics of the rates of hydrogen mass transfer through metallic walls wetted by the eutectic. The experimental results have shown a permeation rate which is much lower than that so far considered in the feasibility study.