The book describes fundamental studies of separation of metal ions using supported liquid membranes (SLM) through the application of selective separation of metal ions such as uranium and thorium. The work is handled by developing mass transfer separation theories to study the single, binary, and multi-component systems. A comprehensive theoretical and experimental study of the different important parameters controlling the process was undertaken. The work reported is unlike the vast majority of previous developments which concentrated on one of the dominant effects limiting the process. First, distribution mechanisms models to describe the chemical environment were developed. Second, solubility mechanisms to describe the transfer of species into the SLM organic phase were encountered. Third, influence of negative viscosity effects on diffusivity of the transfer were considered. Fourth, all the different mechanisms, effects, and models were unified in a generalized mathematical model. The models are very useful for design purposes.