Surface diffusion and adsorption-desorption are the means by which oil is passed through traps in the molecular flow region. Surface diffusion can be excluded by using zeolite (or any equivalent molecular sieve material) on surface paths in the trap. Elimination of the adsorption-desorption mechanism, however, requires somewhat more careful trap design. It is shown that a long isolation time (the length of time that a system will remain at the base pressure after processing for achievement of ultrahigh vacua, which is the usual criterion for evaluation of isolation methods) is a necessary but not a sufficient condition that must be met to insure complete isolation of a system from an oil diffusion pump. It is shown, using field-electron emission microscopy (FEM), that under certain conditions “oil” can be transferred into the vacuum system during bakeout and also under subsequent quiescent conditions. The use of a trap design that is an optical baffle (all internal trap surfaces see only a molecular sieve material, either zeolite or Fiberfrax) precludes “oil” transfer during bakeout, but does not exclude transfer under quiescent conditions. Addition of a Ti getter in-line between the molecular sieve trap and the high-vacuum chamber precludes “oil” transfer under all operating conditions.