A series of experiments was performed to determine the effect of gadolinium as a soluble neutron absorber on the criticality of fuel rod assemblies in uranyl nitrate solution. The gadolinium in the form of Gd(NO3)s was mixed with uranyl nitrate solution. The lattice assemblies were composed of 4.3 wt% 235U-enriched UO2 pellets contained in stainless steel tubes immersed in the uranyl nitrate solution of the same 235U enrichment. Lattice assemblies with center-to-center fuel rod separations of 22.9, 27.9, and 33.0 mm were utilized in this study. In each case, a preset number of fuel rods was positioned in the assembly vessel and uranyl nitrate subsequently added, with the measurement then being of the depth of solution required for criticality. The uranyl nitrate was limited to the fuel-rod-bearing region of the lattice assemblies that were, in turn, reflected with water. Data on integral critical experiments are provided against which calculational techniques can be checked.