Pulsed neutron source measurements have been made on a heterogeneous lattice of plutonium-uranium oxide fuel rods in 258 g(Pu + U)/ℓ nitrate solution containing up to 1.34 g Gd/ℓ. The experimental system on which the measurements were made is not unlike that encountered in fuel element dissolvers. The objectives of the measurements were to demonstrate the use of the pulsed neutron source technique for measuring the effectiveness of a neutron poison in reducing the reactivity of such a system and to determine the kinetic parameter β/l for these systems. Reductions in keff from unity down to 0.64 were observed upon the addition of 1.34 g Gd/ℓ to a critical system. Based on the prompt and delayed critical conditions determined for each gadolinium concentration, a continuous reduction, from $4.35/cm of solution depth down to $0.42/cm, was observed in the reactivity worth of the plutonium-uranium nitrate solution as gadolinium was added to the solution. The values of β/l as a function of gadolinium concentration was observed to vary essentially linearly from 197 to 262 sec−1 as the gadolinium concentration was increased to 1.28 g/ℓ. At the maximum gadolinium concentration of 1.34 g/ℓ, the measurements indicated a β/l value lying above this linear correlation, but not far enough above that it could not be explained by the 0.4% difference observed in the approach-to-critical and the pulsed-neutron-determined delayed critical conditions for this system. The effective delayed neutron fraction, βeff, for these mixed plutonium-uranium systems was calculated to be 0.0033 and was essentially constant over the gadolinium concentration covered. The βeff, calculational technique was subjected to an experimental-calculational verification and was found to be adequate.