The nickel-catalyzed arylphosphonylation with tris(trimethylsilyl) phosphite (TMSP) is applied to brominated high-performance (HP) polymers in weakly coordinating high temperature solvents. At elevated temperatures, this affords halogen-free soluble, phosphonic acid-functionalized polyelectrolytes and an unprecedented control of the degree of phosphonylation (ds(p)). Brominated polysulfone (PS) of various degrees of bromination (ds(Br)) has been phosphonylated with TMSP in dephenyl ether at 200 degrees C in the presence of 1-10 mol-% NiCl2. Upon methanolysis, arylphosphonic PSU with ds(P) up to 223 mol-% is obtained in near quantitative bromine conversion. Catalyst residues are readily removed during methanolysis. This very versatile two-step process affords soluble arylphosphonic polymers for casting tough membrane films. In contrast to sulfonated PSU; the arylphosphonic PSU membranes exhibit improved thermal and chemical stabilities, combined with far lower water swelling in accordance with the demands of polyelectrolyte membrane fuel cell (PEMFC) applications.