Clean Viton and Viton coated with Apiezon‐L vacuum grease were tested for their noncontaminating compatibility with channel electron multipliers (CEMs). The test setup and procedure were the same as those used previously in conjunction with CEM compatibility tests of certain epoxies, solder, and fluorocarbon polymer materials useful for construction of spaceflight sensors. While some CEM gain degradation was noted during exposure to Viton and Apiezon‐L, the present tests indicate that, at least over instrument lifetimes of ∼2×1012counts, these materials should be suitable for (1) preflight space sensor testing systems, (2) hermetic seals for CEM‐based space sensors, and (3) terrestrial CEM‐based instrumentation.