Critical parameters are reported for uranium-solution systems consisting of equally spaced vertical cylinders arranged in a square array resting on the bottom of a 20.3-cm-high square slab tank. Some of these systems were reflected externally. Both the cylinders and the slab contained uranyl-nitrate solution having 490 g of uranium (93.2 wt% 235U)/liter. A system of an 87-cm-high array of sixteen 11.0-cm-diam cylinders on an 11.4-cm-thick solution slab was critical. The slab alone was critical at 12.8 cm. Another critical system was a single 22.4-cm-diam cylinder of effectively infinite height on a solution slab 10.8-cm thick. The 22.4-cm diameter is 93.7% of the critical diameter for an infinite cylinder. Monte Carlo calculations, simulating several typical experimental critical systems, yielded values for keff between 0.958 ± 0.012 and 0.986 ± 0.009.