[en] The authors have studied the isotope dilution gamma-ray spectrometry (IDGS) technique for simultaneous measurements of elemental concentrations and isotopic compositions for both plutonium and uranium in input spent-fuel dissolver solutions at a reprocessing plant. The technique under development includes both sample preparation and analysis methods. For simultaneous measurements of both plutonium and uranium, a critical issue is to develop a new method to keep both plutonium and uranium in the sample after they are separated from fission products. Furthermore, it is equally important to improve the analysis method so that the precision and accuracy of the plutonium analysis remain unaffected while uranium is retained in the sample. To keep both plutonium and uranium in the sample for simultaneous measurements, extraction chromatography is being studied and shows promise to achieve the goal of cosegregation of the plutonium and uranium. The technique uses U/TEVA·Spec resin to separate fission products and recover both uranium and plutonium in the resin from dissolver solutions for subsequent measuring using high-resolution gamma-ray spectrometry. Owing to the fact that the U/Pu ratio is altered during the fission product separation phase, it is necessary to develop a method which could accurately correct for this effect. Such a method was developed using the unique decay properties of ²⁴¹Pu to ²³⁷U and shows considerable promise in allowing for accurate determination of the ²³⁵U concentrations before the chemical extraction