In real hydrometallurgical systems, ion exchange of metal cations is often accompanied by competing processes, such as complex formation. In this paper, we have analyzed influence of complex formation on nickel uptake in a chelating ion exchanger that contains iminodiacetate groups. Both outer-sphere complexation with sulfate and inner-sphere complexation with ammonia were studied at 22 and 80 degrees C. In ammoniacal ammonium sulfate matrix, both these complexes are formed and prediction of nickel uptake becomes challenging. According to the results, nickel uptake can be substantially enhanced by complex formation. Under acidic conditions and at 80 degrees C, proton is complexed by sulfate more effectively than nickel and as a result, the exchange equilibrium favors nickel uptake. High sulfate background concentration further enhances the positive effect. In ammonium sulfate solutions, however, complexation with ammonia is needed to prevent nickel precipitation. The solution is stable near pH 7 and high nickel uptake can be obtained, because competition by H+ is practically absent and Ni2+/NH4+ selectivity is high. Complexation with NH3 thus makes the conditions favorable for Ni uptake rather than affects directly on Ni exchange. All these findings were successfully correlated with a dynamic model including pertinent complexation and dissociation equilibria.