Polycondensation of bis(4-phenoxyphenyl)-1,12-dicarbadodecaborane with aromatic and aliphatic dicarboxylic acids in trifluoromethanesulfonic acid affords a new family of high-MW poly(aryletherketone-carborane)s. The polymers derived from 4,4’-biphenyldicarboxylic acid and 4,4’-oxydibenzoic acid are both essentially amorphous when isolated, but the former undergoes an unusual cold-crystallization process beginning at, rather than above, its glass-transition temperature (267 degrees C), and melts at 386 degrees C. Solutions of the polycarborane derived from 4,4’-oxydibenzoic acid afford high-performance ultrafiltration membranes when gelled in water, with molecular weight cut-off values in the range 9000-16 000 daltons. Poly(aryletherketone-carborane)s undergo quantitative, structure-defined sulfonation in 98% sulfuric acid, resulting in the controlled introduction of two sulfonic acid groups per repeat unit. The resulting ionomers are insoluble in water but readily soluble in the lower alcohols, to give solutions from which thin-film composite nanofiltration membranes may be fabricated. Such membranes exhibit water-fluxes and ion-selectivities which compare favourably to those of many commercial nanofiltration membranes.