Abstract: An extraction agent composition comprising in addition to one or more conventional extraction agents at least one agent, preferably a strong organic acid, affecting the phase interface tension for diminishing the drop size of the dispersed phase and for accelerating the conversion of the material from one phase to another for the separation of nickel (II) and cobalt (II) from each other from aqueous solutions containing both nickel and cobalt. By using said composition the separation can occur regardless of the type of counter-ion, i.e. said separation can take place from sulphate, nitrate and chloride solutions. Basing the separation on a high chloride content or on the necessity of oxidizing the cobalt being present into a trivalent state has been avoided by the addition of a small amount of strong organic acid.