The affect of alkali metal ions (Na+ versus K+) on secondary nucleation of gibbsite (gamma -Al(OH)(3)) from synthetic Bayer liquors has been investigated under seeded, isothermal, batch crystallisation conditions. The nucleation kinetics showed a fourth-order dependence upon AI(III) relative supersaturation and a strong temperature effect. An activation energy of 132 kJ mol(-1), which was independent of alkali metal ion, was calculated. Secondary nucleation and subsequent crystal growth rates however, were greater in sodium than in potassium aluminate solution. The Arrhenius, pre-exponential factor was at least three times larger in sodium than in potassium aluminate solutions at equivalent crystal surface area, similar supersaturation and temperature. The results indicated that secondary nucleation of Al(OH)(3) is a chemical reaction-controlled process which is alkali metal ion-mediated. Na+ ions provide a more favourable pathway than potassium ions for the formation of Al(III)-containing clusters, higher collision frequency between the species and crystal surface, and faster growth of potential secondary nuclei in the solutions.