For the plasma exhaust clean-up of a fusion reactor a process concept based on the hydrogen isotope purification through palladium/silver alloy permeators combined with selective catalytic reaction steps is proposed, which avoids intermediate conversion of impurities into water. To recover tritium from tritiated impurities ammonia is decomposed into the elements inside the permeators; water is reduced catalytically by carbon monoxide into carbon dioxide and hydrogen; and hydrocarbons are cracked into carbon and hydrogen on a nickel catalyst. Experimental results on the reactivity, consumption and regeneration of the catalysts are given. The permeation rate of hydrogen through palladium/silver alloy was found to be largely independent of the impurities CO, CO2, H2O and CH4. Technological requirements in view of NET are discussed.