The kinetics of ytterbium(III) extraction from sulfate medium with Cyanex 923 in heptane has been investigated with a constant interfacial cell with laminar flow, which aimed to identify the extraction regime, reaction zone and rate equations. It was found that the extraction rate of ytterbium(III) increased linearly with stirring speed and specific interfacial area. The activation energy Ea (9.56 kJ mol(-1)), activation enthalpy Delta H-+/- (7.05 kJ mol(-1)), activation entropy Delta S-+/- (298) (-0.31 kJ mol(-1)) and Gibbs free energy of activation Delta G(298)(+/-) (98.3 kJ mol(-1)) were calculated from the dependence of extraction rate on temperature. The experiential rate equations were obtained by investigating the influence of the concentration of various species on the extraction rate. A diffusion regime has been deduced from evidence of the linear dependence of extraction rate on stirring speed and the low value of the activation energy. The liquid-liquid interface is most probably the reaction zone in view of the linear dependence of extraction rate on specific interfacial area, the high interfacial activity and low water-solubility of extractant. Thus the mass transfer rate is controlled by interfacial film diffusion of species. (c) 2007 Society of Chemical Industry.