[en] The effect of methane additions in the welding helium atmosphere of high-pure helium has been studied on the structure and properties of molybdenum welded joint. The change in the impact strength, microhardness and lattice parameters of metal joint in dependence on the methane concentration in the welding atmosphere has been investigated. The microstructure of thin foils, cut out of the metal joint, has been studied. It is shown that optimal content of methane in the helium welding atmosphere is 1x10^-3 vol.%. The increase of the methane content from 1x10^-3 to 1.0 vol.% in the welding atmosphere decreases the impact strength