An experimental and analytical study of the flux distribution of two-region core configurations has been made for the TRX facility. The purpose of this study was to obtain an estimate of the sizes of critical configurations that would yield the same values of the basic reactor parameters in the inner region as a critical core consisting entirely of the inner region material and geometry. Several two-region cores have been constructed and experimental measurements of thermal utilization, resonance escape probability, and fast fission effects have been performed. Slow and fast neutron activation distributions have also been obtained. Two inner regions were constructed utilizing 1.3 w/o enriched UO2 fuel 0.384 in. in diameter and with a density of 10.53 gm/cm3. A third inner region utilized 1.3 w/o enriched uranium metal fuel with a diameter of 0.387 in. Light water served as the moderator and reflector in all cases. The experimental and theoretical results indicate that by utilizing two-region cores, measurements of microscopic parameters can be made for a wide variety of fuel sizes, fuel enrichments, and water-to-uranium volume ratios without the construction of full critical cores for each combination.