The inner synergistic extraction of [tricaprylmethylammonium][sec-octylphenoxy acetate] ([A336][CA-12]) is studied for cobalt and nickel separation. Distribution ratios of [A336][CA-12] for Co2+ or Ni2+ are by far higher than those of tricaprylmethylammonium sulfate ([A336](2)SO4), sec-octylphenoxy acetic acid (CA-12), mixture of [A336](2)SO4 and CA-12 at the acidities from 1 x 10(-5) mol L-1 to 1 x 10(-3) mol L-1. The synergy coefficients of [A336][CA-12] for Co2+ are higher than the synergy coefficients of [A336][CA-12] for Ni2+, which can be used for the Co2+ and Ni2+ separation. Some thermodynamic properties of the inner synergistic extraction using [A336][CA-12] for Co2+ and Ni2+ are also studied, such as influence of temperature, thermodynamic functions, extraction mechanism, separation factors and stripping properties. (C) 2010 Elsevier B.V. All rights reserved.