There has been considerable interest in the use of fixed neutron absorbers (poisons) for criticality control since their use would permit safely handling larger quantities of nuclear materials with reduced probability of criticality. The effectiveness of such absorbers as neutron poisons depends on self-shielding effects which in turn are determined by the magnitude of the absorption cross sections and their variation with energy, the thickness of material, and the neutron-energy spectrum. For the fixed poisons to be considered as either a primary or secondary means of criticality control, their use must be based on a firm knowledge of these effects. To obtain experimental data in this area, the reactivity worth of two such materials, copper and copper containing 1 wt% cadmium, was recently measured in two different energy spectrums and at different thicknesses up to ≈2½ cm. The results of these measurements are presented in this paper and provide a set of clean, well-defined, poisoned critical assemblies that can be used to check calculational techniques and cross-section data., In the relatively thermalizwd neutron-energy spectrum, very little additional absorption was observed in the copper plate at thicknesses greater than ≈2½ cm or in the copper-cadmium plate at thicknesses greater than 1 cm. At thicknesses greater than these, self-shielding precluded any additional absorption, and the change in reactivity was due almost entirely to the additional void being introduced into the system by the poison plates., In the relatively fast neutron energy spectrum, neutron absorption was observed to continually increase with plate thickness for both sets of plates. However, in this spectrum the void effects, caused by the presence of the poison plates, had a greater reactivity worth, over the thickness range covered, than the neutron absorption., In either spectrum, the 1 wt% cadmium in the copper contributed significantly to the neutron absorption. Of course, the cadmium was found to be worth much more in the thermalized spectrum.