The residual gases present in a DC-705 oil‐diffusion‐pump uhv system have been determined as a function of various trapping conditions. The system was equipped with a metal 2-in. oil‐diffusion pump in series with a specially designed trap in which zeolite was used. The configuration of the trap permitted the trapping zone to be immersed in cryogenic liquids, e.g., LN2. The effectiveness of the trap was determined as a function of temperature. A comparison of the residual gases is made for six conditions, including three trapping temperatures, while operating with zeolite and without zeolite.