The rate at which contamination forms on surfaces exposed to electron beams in reflection electron diffraction has been investigated for electropolished single crystals of copper and cleavage faces of alkali halide crystals by observing the decrease of the intensity in the diffraction pattern. It was found that a cold chamber at liquid nitrogen temperatures surrounding the specimen reduces the contamination rate on copper by an amount directly proportional to the solid angle through which the specimen is exposed to the vacuum system, and at the smallest aperture lowers the rate by as much as 2 orders of magnitude. Color centers were produced in the alkali halide crystals exposed to the electron beam. The resultant imperfections in the crystal lattice apparently affected the diffraction spot intensity, making measurement of contamination rates by this method unsatisfactory.