We report on the effect of nanoscale morphology on transport of ethanol/water mixtures through block copolymer membranes. In particular, we show that ethanol selectivity and overall flux can be optimized by varying the size of block copolymer domains at fixed composition. Experiments were conducted on two separate systems using polybutadiene (PB) and polydimethyl siloxane-g-polymethylmethacrylate (PDMS-MA) as the transporting blocks, using an 8 wt% ethanol/water mixture as the feed. The domain spacings for PB- and PDMS-MA-based samples were varied from 19 to 55 nm and 28 to 70 nm, respectively, at fixed compositions. The membrane separation factor increases with domain spacing for both membranes. In the case of the PB-based system, the smallest domain size system with d = 19 nm was water selective while those with larger domain spacings were ethanol selective. The total flux through the PB-based 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 PDMS-MA-based membranes does not depend on domain spacing. (C) 2011 Elsevier B.V. All rights reserved.