Technical Abstract: We report on the effect of block copolymer domain size on transport of liquid mixtures through the membranes by presenting pervaporation data of an 8 wt% ethanol/water mixture through A-B-A and B-A-B triblock copolymer membranes. The A-block was chosen to facilitate ethanol transport while the B-block was chosen to provide the membrane with the necessary mechanical strength. Separate experiments were conducted on polystyrene-b-polybutadiene-b-polystyrene (SBS) and (polydimethyl siloxane-g-polymethylmethacrylate)-b-polycyclooctene-b-(polydimethyl siloxane-g-polymethylmethacrylate) (DCD) membranes. The SBS samples contained hexagonally packed cylindrical PS domains in a PB matrix. The DCD samples contained alternating lamellae of PDMS-MA and PCOE. The PB and PDMS-MA domains serve as the alcohol transporting domains in the SBS and DCD systems, respectively. The domain spacings for SBS and DCD samples have been varied from 19 to 55 nm and 28 to 70 nm, respectively, at fixed composition by varying the total molecular weight of the copolymers. In both SBS and DCD, the membranes comprised randomly oriented domains. The membrane separation factor increases with domain spacing for both SBS and DCD membranes. In the case of the SBS system, the smallest domain size system with a d=19 nm was water selective while those with larger domain spacings were alcohol selective. The total flux through the SBS membranes increases with increasing domain spacing until it reaches a maximum at a domain spacing of 39 nm in spite of the fact that the stiffness of the polymers increases monotonically with increasing domain spacing. In contrast, the total flux through the DCD membranes does not depend on domain spacing. Water and ethanol uptake experiments in conjunction with a simple model that equates the chemical potential of ethanol and water in the membrane and liquid were used to determine the affinity of the SBS membranes to ethanol and water. The calculated sorption selectivity, defined as the ratio of the mass uptake of ethanol to water in the membrane divided by the same ratio in the liquid was less than one for the SBS system with d=19 nm and greater than 1 for d=39 nm.