Hydrocarbon transformations and separations are what define refinery operations. Polymeric membranes for reverse osmosis in water purification have been in use since the 1960's, but polymeric membranes for molecular-scale separations in non-aqueous systems have just started to see industrial applications in the 1990's. This is in part because there is strong interaction between polymers and small hydrocarbon molecules. This interaction provides the potential for hydrocarbon separations, but also provides for technical challenges not considered in aqueous applications. Therefore, one key step to investigation of new applications for membrane separations is choice of membrane material. These materials can range from soft rubbery polymers such as polysiloxanes to high temperature rigid polymers such as polyimides. There is an interesting balance between choosing a polymer, which itself is hydrocarbon based, and achieving an interaction with a hydrocarbon mixture, which simultaneously does not impair membrane performance. This is especially true when looking at long-term applications where multi-year performance of installed membrane systems is desired. Equally important to separation quality are the process conditions under which the separation is performed, with choices ranging from pervaporation to organic solvent nanofiltration. Also important are the qualities of the hydrocarbon mixtures, in whether the separation will be based on molecular weight or physical property differences between the molecules. Experiments with hydrocarbon mixtures taken from applications in refining, chemical, and pharmaceutical industries illustrate some of the remarkable separations that have been accomplished. In addition, looking at transport rates for molecules calculated from experimental data using solution-diffusion equations are found helpful in developing new applications.