A chemically based thermodynamic model to pr edict the distribution coefficient has been developed for the Au(CN)(2)(-)-Primene JMT-Cyanex 925 system. The predictive model makes use of the extraction mechanism, speciation in the organic phase, and the equilibrium constant for the extraction reaction. The distribution coefficient of gold can be predicted by the equation D-Au = K＇[H+], where K＇ is an effective extraction constant and [H+] is the proton concentration of the aqueous phase at equilibrium. Excellent agreement between the experimental data and the predicted value was obtained. Furthermore, for the present extraction system the equilibrium isotherm was determined at 20 degrees C.