Solvent-cast films of poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymers and block ionomers were analyzed using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Four block copolymer samples with center block molecular weights of 52,000 g mol(-1) and PS volume fractions (phi(PS)) ranging from 0.17 to 0.31 were studied. Block ionomers were produced by lightly sulfonating the PS outer blocks of the PS-PIB-PS block copolymers. Sulfonation levels were 1.5 and 3.4 mol%, and sodium and potassium counterions were studied. All samples exhibited hexagonally packed cylinders of PS within the PIE matrix. Cylinder spacing was in the range 32-36 nm for most samples, while cylinder diameters varied from 14 to 21 nm. Pored analysis of the scattering data indicated the presence of isolated phase mixing and sharp phase boundaries. (C) 2000 Elsevier Science Ltd. All rights reserved.