The extraction conditions for the removal of cobalt(II) from chloride aqueous solution were studied by using supported liquid membrane. The effects of four factors such as the acidity of feed solution (pH), Cyanex301 concentration, cobalt ion concentration in the feed phase as well as sulfuric acid concentration in the stripping phase were investigated by using central composite design which is a subcategory of response surface methodology. The optimum conditions obtained for pH, Cyanex301 concentration, initial cobalt concentration in feed solution, and sulfuric acid concentration were 6, 1 mol/L, 98.86 mg/L and 1.26 mol/L, respectively. The percentage of cobalt extraction equal to 48.21% obtained from the predicted model was in agreement with the experimental data (45.84%), after 3.5 h in the experimental conditions. In addition, the other factors such as the types of solvent as a diluent, stripping solution, and the types of extractants and mixtures were studied at the optimum point. The results showed that the percentage of cobalt extraction achieved was 46.85% within 3.5 h, in the extraction conditions (Cyanex301 as an extractant, kerosene or chloroform as a diluent and the mixture of sulfuric acid and nitric acid as the stripping phase). (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.