The synergistic effect of Cyanex 302 on solvent extraction of nickel and cadmium from sulfate solution with D2EHPA diluted in kerosene has been investigated with the aim of increasing separation efficiency. Experiments are carried out using the sole D2EHPA and also D2EHPA–Cyanex 302 mixtures in different ratios. As a result, satisfactory separation has been found with a Cyanex 302 to D2EHPA ratio of 0.1 M:0.5 M. The extraction equilibrium constants, enthalpy change, and entropy change of the cadmium extraction are also determined. The stoichiometric coefficient of cadmium for the sole D2EHPA and three different D2EHPA to Cyanex 302 ratios has been determined by applying the slope analysis method. The experimental data of cadmium extraction have been employed to model the distribution coefficient of cadmium by multiple linear regression.