[en] The feasibility of welding thin-walled high vacuum devices has been evaluated with the goal of defining a process capable of consistently producing welds which are leak tight and which possess sufficient fracture toughness so as not to develop leaks later in life. The principal problem in welding molybdenum is related to a high propensity for intergranular fusion zone cracking due to grain boundary oxide films. The oxygen content of commercially available grades of molybdenum is sufficient to embrittle grain boundaries of typical coarse-grained fusion zones. The overall experimental program involves three approaches to improving the ductility of the molybdenum welds: (1) grain refinement, (2) reduction of the oxygen content of the fusion zone, and (3) redistribution of oxides. The first two approaches are the subject of this paper