The viability of using palladium alloy membranes in cascade to separate the hydrogen, deuterium and tritium produced in a fusion reactor is discussed. Values of the permeation separation factor are estimated for Pd-20%Ag from available data and corresponding estimates for the necessary number of stages and the total membrane area are given for the required isotopic purification. The results demonstrate the advantage of minimizing the operating temperatures and pressures and indicate that the method is perfectly feasible. The use of Pd-8%Y will probably even further improve the performance.