Deposition of molybdenum by vaporization from simple hairpin filaments has been found to reduce pressures in both unbaked and moderately baked stainless steel vacuum systems to 4×10−10mm Hg. A deposit of 300 mg from a single filament, 0.050 in. in diameter and 6 in. long, has maintained an unbaked 85‐liter volume at pressures below 10−9mm Hg for over 40 hr with the aid of a small well‐trapped diffusion pump. For a substrate area of 8×103cm2, the initial pumping speed of a molybdenum deposit was found to be as high as 105liters/sec for hydrogen and 8×104liters/sec for deuterium. The sticking probability for either hydrogen or deuterium on this deposit is estimated at 0.3.A similar system, baked at 200°C for several days while pumped by an oil‐free ion pump, attained 2×10−10mm Hg with molybdenum evaporation. When the ion pump was valved off, the pressure in this 75‐liter system remained at 2×10−10mm Hg for two weeks with no external pumping.