AbstractThe thermal-neutron diffusion constant was measured in a plexiglas system with empty channels, using the pulsed-neutron-source technique. From separate sets of measurements on rectangular blocks, the diffusion constants parallel and perpendicular to channels were determined, whereas the measurements on cubes give the average diffusion constant. The results are compared with existing theoretical estimations. It is found that the average diffusion constant is well below the theoretical predictions of Behrens. On the other hand, the difference between the parallel and the perpendicular diffusion constant is almost as large as predicted theoretically.