The thermal-neutron flux depression was measured in cylinders containing materials that mock up various enrichments of uranium dioxide. Six chemical compounds were mixed in various proportions to match the macroscopic absorption and scattering cross sections and the average cosine of the scattering angle for UO2. Cylinder diameter was varied from 0.635 to 1.27 cm (0.25 to 0.50 in.) and the mockup enrichment was varied from 5 to 100%. For enrichments up to 20%, the measured flux depression was greater than that predicted by simple diffusion theory. At higher enrichments, diffusion theory significantly overestimates the flux depression.