Experiments were performed to establish the effect of a soluble neutron absorber (gadolinium nitrate) on the criticality of plutonium nitrate solutions. The solutions contained plutonium at concentrations of ∼116 g Pu/liter and at ∼363 g Pu/liter. Measured quantities of gadolinium nitrate were mixed with these solutions to produce changes in critical solution height within a 24-in. -diam water-reflected cylinder. Gadolinium concentrations up to 20.25 g Gd/liter were used and the effect determined through the observed change in height. Monte Carlo calculations were used to compute the criticality factors (keff’s) for each of the measured critical configurations. The computed factors were below unity in each case (largest departure about 2% less than unity). The gadolinium proved to be an effective neutron absorber. Its effectiveness decreased significantly, however, in the higher plutonium concentration and faster neutron spectrum. Although comparable values of k∞ were computed (1.603 and 1.503) for the two plutonium concentrations in the experiments, the calculations show 2.4 g Gd/liter would be required to reduce k∞ to unity in the first case, whereas about 72 g Gd/liter would have been required in the second (316 g Pu/liter solution). Curves were prepared showing the computed quantities of gadolinium required to reduce k∞ to unity as a function of plutonium concentration. Also included are computed critical radii for infinitely long cylinders of plutonium nitrate solution for several different gadolinium concentrations. There was no evidence of chemical instability (or precipitation) of the gadolinium in the plutonium nitrate solution during the course of the experiments and over a 1-mo long test (a question of concern in using soluble poison for criticality control).