Carbon molecular sieve separation membranes have broad applications in the field of various separation processes. To attain higher permeation and higher selectivity than the normal separation efficiency, pore structure should be controlled. It is then a requisite that this pore structure be characterized. For industrial applications of carbon molecular sieve membranes, supported membranes are considered because of the brittle characteristic of standalone carbon membranes. The isotherm adsorption technique is common for characterizing carbon molecular sieve membranes, but it is not applicable for composite membranes. To understand the relationship between pore structure and separation performance of carbon molecular sieve composite membranes, the positron annihilation technique for characterizing the variation in the pore structure with the membrane depth was applied in this research. Ortho-positronium (o-Ps) lifetime is a means of probing the free-volume/pore by the positron annihilation technique, but it cannot be formed in carbonaceous materials with a strong negative charge. As such, it is not possible to get any pore information based on the o-Ps for carbon molecular sieve membranes. Hence, a τ2-r semi-empirical equation of positron lifetime was developed and calibrated in this research; it is according to the wave function of infinite spherical potential well postulated by Tao in 1972. Temperature-dependent and pressure-dependent PALS was utilized for measuring positron lifetime (τ2) and o-Ps lifetime (τ3) data for thermoplastic and thermosetting polymers. Free-volume size calculated by o-Ps lifetime was correlated to positron lifetime, which had no quenching and inhibition effects. Positron lifetime data on sizes of well-known molecular sieves and zeolites from the literature were collected. The calibrated semi-empirical τ2-r equation was obtained by regression analysis with these data. This equation enabled the calculation of free-volume/pore size in systems where no o-Ps was observed, such as polyimide and carbonaceous materials. Remarkable advances have been made in computing the free-volume/pore size in polyimides and carbon molecular sieves. The calibration of τ2-r semi-empirical equation enhanced our ability to analyze the microstructure of carbon molecular sieve separation membranes. Thus, this semi-empirical equation was utilized for analyzing the change in the microstructure of the carbon molecular sieve membranes with the depth at different carbonization conditions. During the preparation of the carbon molecular sieve separation membranes, different carbonization temperatures induced different degrees of pyrolysis and carbonization, which influenced the final structure of the membranes. Pyrolysis and heat dissipation produced volatile substances, leading to a non-symmetric membrane structure. The slow positron technique coupled to Doppler broadening energy spectroscopy was utilized to determine the effect of the mass transport on the structure of the resultant membrane and the generated non-symmetric structure, to understand the relationship between carbonization conditions and the microstructure, and to evaluate the gas separation performance. Based on test results at 900℃ for the carbon molecular sieve separation membranes, the S parameter curve obtained from the Doppler broadened energy spectra of the annihilation radiation showed a gradual increase as the temperature increased. This result provided further support that the carbon molecular sieve membranes had an asymmetric structure at specific carbonization conditions. With the use of a variable monoenergy slow positron beam, a three-layer structure was obtained from the membrane carbonization at 900℃ with no holding time: dense surface layer, transition later, and inner layer. When the holding time was prolonged, the layer thickness decreased and the pore volume in each layer shrunk, resulting in lower gas permeability and higher selectivity. The calibration of the τ2-r semi-empirical equation and the resulting analysis showed its powerful function to characterize carbon molecular sieve membranes. After tuning and analyzing the microstructure, the resultant membranes were applied to membrane separation processes. To widen the application of the carbon molecular sieve membranes, these membranes with molecular sieve character were adopted to dehydrate an aqueous ethanol solution at 70℃. The pervaporation process was examined and compared with vapor permeation results. The τ2-r semi-empirical equation developed in this research was utilized to estimate the free-volume/pore size in Kapton and carbon molecular sieve membranes fabricated at different carbonization temperatures. Results of pore size measurements, TGA-MS, ATR-FTIR, XPS, and sorption tests with pure/mixed solutions revealed the physical/chemical change of the polymers subjected to carbonization. The carbon molecular sieve membranes prepared at 700℃ showed the best ethanol dehydration performance, which could be attributed to the optimum conditions affecting pore shrinkage and form. The membranes showed high performance, as permeance and water concentration in permeate were 1199 g/m2hr and 98.7%, respectively. In summary, this research successfully developed a semi-empirical equation of τ2-r, which was effective in characterizing the average free-volume/pore size in polyimide and carbon molecular sieve separation membranes, the variation in the microstructure with the depth of the membranes, and the relationship between the structure, the pervaporation, and the gas separation efficiency. Results from this research could succeed in transforming the key technique necessary for widening applications of carbon molecular sieve separation membranes.