Development of a solvent extraction process with a single solvent for separating Co and No has the advantages of succinct process and recyclability of components. Main problem with the known specific organic extractants are they are expensive and have limited availability. Previous research on Co/Ni solvent extraction systems have employed two main strategies; modification of organic phase and adjusting the conditions of aqueous phase. The aim of present work was to study the effect of tartrate ion, a carboxylate ligand, in the separation conditions of Co and Ni via D2EHPA. Preliminary experiments were conducted to examine the effect of tartrate ion on the extraction curves of Co and Ni. Fourier Transform Infrared Spectroscopy (FT-IR) was employed to elucidate whether metal-organic complexes containing Co/Ni and tartrate were formed. For the estimation of the stoichiometric coefficients, slope analysis method was applied. Response surface methodology (RSM) with a central composite design (CCD) was used to quantify the effect of pH, tartrate concentration and temperature on the extraction process of Co and Ni and also to develop second order polynomial models for optimization purposes. (C) 2014 Elsevier Ltd. All rights reserved.