A series of criticality experiments have been completed with plutonium nitrate solutions made up from extremely high burnup fuel (239Pu isotopic concentration <½ total Pu). The measurements were performed on a large, 61-cm-diam, water-reflected, cylindrical vessel. The critical experiment data were analyzed by means of the KENO Monte Carlo code utilizing both ENDF/B-H and -III cross sections; the computed criticality factors were in the range of 1.6 to 1.9% above unity. The effects of the various heavy element isotopes on criticality were analyzed. The results show the importance of properly accounting for, and treating, the effects of each of the various isotopes in computing criticality. Even the presence of as little as 1% of 241 Am can cause a change in the reactivity of the solutions used in these experiments by ∼1%.