We have investigated a series of Fe-Mo-C-B and Fe-Mo-Ge-C-B iron-base brazing filler metals for joining molybdenum. Our development method can be adapted for many other brazing requirements, and the filler metals can have other applications. The filler metals exhibited excellent flowability between 1050 and 1200°C. Each brazed molybdenum to itself, and at least two brazed molybdenum to austenitic stainless steels, to nickel, and to carbon steels. Molybdenum-to-wiolybdenum brazes possessed shear strengths exceeding 30 000 lb/in.2 (207 MPa) at room temperature, with accompanying elongations exceeding 10%. At 650°C the strengths (depending on composition) were 18 000 to 29 000 lb/in.2 (124 to 200 MPa), with elongations exceeding 42%. These alloys showed outstanding corrosion resistance when exposed to either liquid sodium or molten fluoride salts at 600 to 700°C. In addition, the Fe—15% Mo—5% Ge—4% C—1% B and Fe—15% Mo—4% C—1%B (by weight) brazing filler metals had satisfactory corrosion resistance when exposed to static or flowing liquid bismuth at 600 to 700°C. When tested in lithium for 800 h at 800°C, the germanium was preferentially leached from the Fe—15% Mo—5% Ge—4% C—1% B braze. However, with further development the iron-base filler metals do appear promising for lithium service.