Critical-experiment data are presented on a heterogeneous lattice of fuel rods comprised of uranium and plutonium oxides, clad with stainless steel, and moderated with (U + Pu) nitrate solution, a condition not unlike that encountered in a fuel-element dissolver operation. The effect of a soluble neutron absorber (gadolinium nitrate) on the criticality of this type of system was also examined for its possible use as a method of criticality prevention and control during the dissolution step. The results provide data for code validation, an essential requirement on complex systems such as this, if the calculations are to be utilized to prescribe subsequent control limits under similar or related conditions in fuel processing. Experiments indicate (for the very limited data presented) that a heterogeneous system composed of these fuel rods in water can have a larger buckling than the fuel in the dissolved state. The question is, whether a fuel rod of a size different from that used in these experiments, immersed in fissile-bearing solutions, might have a still higher buckling (and smaller critical size) than the highest achievable buckling for fuel rods of optimum diameter and spacing in water. This important consideration regarding the criticality safety aspects of dissolvers must be examined in each case. The results of calculations of these systems with the KENO Monte Carlo code utilizing ENDF/B-III cross sections are presented.