Critical experiments were performed at the Pacific Northwest Laboratory’s Critical Mass Laboratory in 1987 and 1988 with a heterogeneous array of mixed-oxide (MOX) fuel pins immersed in mixed plutoniumuranium nitrate solutions. The 996 fuel pins, on a 1.40-cm-square pitch, were configured in a cylindrical array. The solution heavy metal concentrations ranged from 4 to 468 g/ℓ and had a Pu/Pu+U ratio of 0.2. Critical experiments were also performed with gadolinium added to the fissile solution. These experiments were designed to simulate conditions in a MOX fuel dissolver, where fuel lumps are moderated by aqueous solutions containing fissile nuclides, with and without a soluble neutron poison. For the experimental conditions examined, it was determined that the critical size of the system increased as the heavy metal concentration increased. The criticality data were used to validate two versions of the SCALE computer code system and the 27-energy-group cross-section library, derived from the Evaluated Nuclear Data File B Version IV. The calculational results indicate that SCALE-2 has some difficulty in modeling these systems. Modifications in SCALE-4 have led to more accurate keff results.