The PCTR is a seven-foot cube of graphite with a large cavity, 2 x 2 x 3 ft, located at its center. It is made critical by enriched uranium which is distributed on the boundary of the central cavity. One end of the assembly, 2 x 7 x 7 ft, is mounted on a movable cart, and can be moved away from the reactor proper allowing access to the central test region. The infinite medium, thermal neutron multiplication factor, k∞, of a multiplying material is obtained by determining the amount of thermal absorber, which, when inserted with the multiplying material into the central region of the PCTR, will change neither the reactivity of the assembly nor the energy distribution of neutrons in it. The design of the reactor and the method used for determining this absorber mass are discussed and results for two graphite-natural uranium lattices are presented.