Gravimetric measurements of diffusion-pump backstreaming rates were made above liquid-nitrogen-cooled traps. Experiments were made of over 500-h duration, with 6-in. and 35-in. pumps, with and without a water-cooled baffle and a creep barrier. The surfaces collecting the backstreaming oil were cooled to liquid-nitrogen temperature to prevent errors due to re-evaporation. Design compromises are discussed as related to the requirement of maximum pumping speed and minimum backstreaming. The results indicate that opaque water-cooled baffles have substantial redundancy in the design. A combination of a water-cooled partial baffle, a liquid-nitrogen trap, and a special cold cap over the upper nozzle is described, which permits important improvements in system speed without significant increase in the backstreaming rate.