Homopolymers containing ionic moieties such as sulfonic acid groups are suitable as fuel-cell, water-purification and ion-exchange membranes or, alternatively, as electroactive or otherwise stimuli-responsive soft materials. Block ionomers constitute a class of ionomers in which (i) long sequences of repeat units are sufficiently incompatible to microphase-separate and (ii) at least one sequence possesses ionic groups along its backbone. In this study, the midblock of a model poly(p-tert-butylstyrene-b-styrene-b-p-tert-butylstyrene) (TST) triblock copolymer has been selectively sulfonated to different degrees by reaction with acetyl sulfate. Self-assembly of the unaffected endblocks prevents the resultant block ionomers from dissolving in water and imparts water-swollen ionomer films with sufficient mechanical integrity to remain intact. The degree of sulfonation is controlled by varying the concentration of acetyl sulfate, as confirmed by spectroscopic analyses. An increase in ionic content promotes a reduction in thermal stability due to sulfonic acid cleaving, but a significant increase in water sorption that increases with increasing temperature. Although the parent TST copolymer is disordered according to small-angle X-ray scattering, moderate levels of midblock sulfonation induce microphase ordering and simultaneously increase the intermicrodomain distance due to swelling considerations.