The results and analyses presented are from the latest series of experiments in a continuing program for determining the critical parameters of plutonium mixtures having concentrations typical of wet powders, precipitates, slurries, and polymers. Previous measurements in this program were made on 15 H/Pu fuel having 240Pu isotopic concentrations of 2.2 and 8.08 wt% and on 5 H/Pu fuel having a 240Pu isotopic concentration of 11.46 wt%. This latest series of experiments was conducted with fuel in which the 240Pu isotopic content has been increased to 18.35 wt% and the H/Pu atomic ratio decreased to essentially zero. The minimum critical slab thickness for a water-reflected homogeneous PuO2-water system of this composition was determined to be 2.31 ± 0.06 cm as compared to only 1.15 ± 0.03 cm for Pu-water. Thus, having the plutonium in an oxide form at this degree of moderation results in an increase of about a factor of 2 in the critical thickness. For spherical geometry, the difference in critical mass between PuO2 and Pu systems is about 2. Also, in the fast neutron spectrum of this fuel, the percent change in spherical critical mass per percent change in 240Pu content was determined to be 2.07 for the reflected case and 1.88 for the bare case. In general, results from these experiments indicate that the values for the critical sizes and masses of plutonium given in references such as TID-7028 should be increased for the highly concentrated systems.