A poly(phenylene-co-arylene ether ketone) with five quaternary ammonium (QA) groups on a phenyl unit (PQA-PPAEK) was prepared to investigate the alkaline stability of multifunctional QA groups in an anionic-exchange membrane fuel cell. The properties of the PQA-PPAEK membrane were compared with those of a polymer with the same backbone but only one quaternary ammonium group on the phenyl unit (QA-PPAEK). Due to the relatively high feeding ratio of hydrophilic to hydrophobic moieties, continuous ion clusters formed with the QA-PPAEK, which resulted in much higher hydroxide conductivities over the entire temperature range than with PQA-PPAEK. Consequently, the highest power density achieved with QA-PPAEK was 81.2 mW cm(-2), whereas it was 55.4 mW cm(-2) with PQA-PPAEK. A kinetic study showed that the degradation rate of the QA groups in the locally dense PQA-PPAEK was nearly twice as high as that in QA-PPAEK, which was consistent with phenomena observed in the durability test. Moreover, the corresponding model monomer showed excellent stability under alkaline conditions, whereas PQA-PPAEK degraded much faster, with only 50% of the initial conductivity remaining after 1000 h of testing.