A cryogenic diffusion pump has been developed and tested. It uses the main constituent of the fusion reactor exhaust gas, i.e. D-T, as a working fluid in a diffusion pump operating at low temperature, to separate and compress the minor constituent 4He. A prototype was tested using mixtures of deuterium with 1%, 2% and 4% helium. As the deuterium is pumped by cryocondensation at 4K, the entrained helium is staying in the gas phase, is compressed and leaves the pump outlet at pressures up to 80% of the total inlet pressure. Compression ratios of up to 80 (for 1% helium) and inlet pumping speeds ranging from 1 to 2m3 s-1 for operating pressures between 3 and 10 Pa have been measured. Deuterium carry-over into the helium exhaust was below the detection limit of the equipment. A design is presented incorporating the cryogenic diffusion pump in a complete fusion reactor forevacuum system, including the separation of impurities from hydrogen isotope mixtures.