Computer aided design and simulation of a large class of separation processes involving many components (e.g., distillation and absorption) is currently carried out under the assumption that the various components are always at exact thermodynamic equilibrium at every part of the equipment. This is not always true, and frequently the separation is controlled by the rate of transfer of components from one fluid to another. Yet, presently, models based on mass transfer are not available for use in design and simulation. This proposal outlines a research program to develop and test such rate-dependent models. If successful, the results will have a potential impact on a large number of multicomponent separation operations. Further, the results of this study will be useful in evaluating the current status of design concepts based on rate-controlled separation processes and in determining whether currently available experimental data are sufficient to proceed with the development of design procedures such as the ones suggested in this proposal.