The TRUEX solvent extraction process is being developed to remove low concentrations of transuranic (TRU) elements from high-level and TRU wastes currently stored at US Department of Energy sites. Cadmium is an important constituent of some of these wastes, especially at the Idaho Chemical Processing Plant. Cadmium distribution ratios were measured by using a radioactive Cd tracer under a variety of aqueous-phase compositions to elucidate the mechanism of cadmium extraction by TRUEX-NPH solvent at 25 degrees C. A model for predicting cadmium distribution ratios was developed as a function of the thermodynamic activities of water and nitrate ion. The Generic TRUEX Model (GTM) was used to calculate these activities. The GTMs calculation of activity coefficients is based on a Bromley method.