The application of the Rotating Diffusion Cell (RDC) to the study of solvent extraction kinetics has been described. In the RDC the two phases are brought into contact at the surface of a membrane filter. The filter la rotated on a cylinder, establishing well defined rotating disk hydrodynamics on either side of the filter surface- It is therefore possible to characterize precisely the diffusional processes of the reactant and product species on the aqueous and organic sides of the interface. The area of interfacial contact may be varied without affecting the diffusional processes. It is the characterization of the diffusional processes and the ability to vary the area of interfacial contact which makes this technique very useful in solvent extraction kinetic studies. A few experiments from the cobalt-HEHEHP system have been described to illustrate the use of the RDC.