Typically, gas separation with membranes concentrates on mixtures profusely studied, such as CO2/N2, CO2/CH4, H2/CO2, O2/N2, H2/N2, He/CH4, H2/CH4, and certain hydrocarbon mixtures, not addressing some non-common, rare but technologically interesting mixtures. However, membrane technology has the potential to address the efficient separation of such rare mixtures, not reported in the current upper bounds basically due to the lack of experimental studies. This review devotes to the membrane application to mixtures including the following: H2/He, H2/D2, 3He/4He, He/Ne, Kr/Xe, CO2/He, and traces from the air. In most cases, the membrane performance has only been studied from a theoretical point of view suggesting that a lot of work is still needed to eventually think of an industrial operation. In any event, there is no doubt about the fact that the advantages that the membrane technology presents for the most studied and better-established gas separations will contribute to solving the future gas separation of these rare mixtures.