Ethylene is the most important organic molecule with regard to production volume. Therefore, the energy spent in its separation processes, based on old-fashioned distillation, takes approx. 33% of total operating costs. Membranes do not require significant thermal energy input; therefore, membrane processes may separate hydrocarbons cheaply and just as reliably as distillation columns. Olefin/paraffin separations are the future targets of commercial membrane applications, provided high-performing materials become available at reasonable prices. This thesis addresses the development of advanced carbon molecular sieve (CMS) membranes derived from intrinsically microporous polymers (PIMs). Chronologically, Chapter 4 of this work reports the evaluation of PIMs as potential ethylene/ethane selective materials, while Chapters 5 to 7 propose PIMs as carbonization precursors. The gravimetric sorption studies conducted in this work regarding both the polymers and their heated-derivatives revealed that this separation is entirely controlled by diffusion differences. The pristine polymers examined in this study presented BET surface areas from 80 to 720 m2g-1. Furthermore, the effect of using bromine-substituted PIM-polyimides elucidated a boost in ethylene permeability, but with a significant drop in selectivity. The hydroxyl functionalization of PIM-polyimides was confirmed as a valuable strategy to increase selectivity. Functionalized PMDA-HSBF is the most selective polyimide of intrinsic microporosity known to date (= 5.1) due to its hydrogen-bonded matrix. In spite of their novelty, pristine PIMs based on the spirobisindane moiety were not tight enough to distinguish between the 0.2 Å difference in diameter of the ethylene/ethane molecules. Therefore, they did not surpass the upper bound limit performance of known polymeric membranes. Nevertheless, the carbons derived from these polymers were excellent ethylene/ethane sieves by virtue of their narrow and tight pore distribution around the 3.6- 4.4 Å range. PIM-based carbons were typically 10 times more permeable than their corresponding low free-volume analogues treated after the weight-loss of the sample reached a plateau. Furthermore, carbons derived from PIM-6FDA-OH and PIM-6FDA at 800 ºC were as ethylene separating efficient as their lower free-volume counterparts. The pore sintering mechanism that takes place above 600 ºC during the carbonization procedure of these films reduced the entropic freedom of the molecules, as was observed from separation factors of up to 25 under pure-gas conditions and 2 bar of pressure— The best performing CMS membranes reported to date for ethylene/ethane separation. The mixed-gas separation of 1:1 binary ethylene/ethane mixtures revealed a significant decrease of the pure-gas measurements due to a carbon matrix dilation effect. This localized ultramicroporous dilation caused the ethane permeation rate to increase monotonically as the pressure rose to realistic operating values. Nevertheless, the CMS obtained from PIM-6FDA and PIM-6FDA-OH surpassed any diffusion-controlled polymer or carbon that has been reported to date.