A transfer cell of constant interfacial area was utilized to measure the rate of chromium (VI) extraction from aqueous solution with tri-n-octylphosphine oxide (TOPO) dissolved in kerosene at 25 degrees C. The effects of agitation, interfacial area, extractant concentration, pH of aqueous solution and concentration of chromium(VI) on the extraction rate were discussed. It was found that the extraction rate in the present system was controlled by interfacial chemical reactions. On the basis of an extraction mechanism including the adsorption of TOPO on the organic-aqueous interface, rate equations were derived taking into account various chromium(VI) species in aqueous solution. By best fitting the rate equations with experimental data using Rosenbrock method, the forward and backward reaction rate constants of chromium(VI) species with TOPO as well as the equilibrium constants for the adsorption of TOPO on aqueous-organic interface and for the desorption of Cr (VI)-TOPO complexes from the interface were obtained. The kinetic model proposed in this work was successfully correlated with the experimental results.