Critical experiments were conducted with organic- and water-moderated arrays of mixed plutonium and uranium oxide fuel pins. The organic solution consisted of 32 vol% tributyl phosphate and 68 vol% normal paraffin hydrocarbon. The square lattice pitches ranged from 0.761 to 1.935 cm. A direct comparison between the reactivity of the two systems was not possible, because most of the experiments conducted with water were performed ∼7yr before the organic solution experiments. At the two lattice pitches where a direct comparison could be made, based on the number of fuel pins required for criticality, it was determined that the reactivity of the water-moderated system was the same or slightly higher than the organic-moderated system. These data were used in calculational studies performed independently at the Oak Ridge National Laboratory (ORNL) and the Power Reactor and Nuclear Fuel Development Corporation of Japan (PNC) with the KENO-IV computer code. A 27-energy-group cross-section library derived from the Evaluated Nuclear Data File B-Version IV (ENDF/B-IV) was used in both studies. The results of these analyses are in good agreement with the experimental results with calculated keff’s ranging from 0.991 to 1.014. The average calculated keff’s based on the ORNL and PNC analyses were 1.001 and 1.004, respectively. No trends in calculated keff with any parameters were identified.