Poly(arylene ether sulfone)s have been functionalised with alkyl side chains carrying phosphonic acid and bis(phosphonic acid) in order to investigate their properties as membrane materials. The phosphonated and bisphosphonated polymers were synthesised via lithiation of poly(arylene ether sulfone)s, followed by reactions with triethyl 3-phosphonopropionate and tetraisopropyl vinylidenediphosphonate, respectively. Flexible and mechanically tough acidic membranes were cast from solution after hydrolysis of the ester groups. The significantly higher acidity and acid concentrations of the membranes containing the tetraprotic bis(phosphonic acid) led to higher conductivities in comparison to the membranes containing the diprotic phosphonic acid. Membranes containing 1.7 mmol of bis(phosphonic acid)/g dry polymer absorbed 28 wt% water when immersed at room temperature, and a conductivity of 25 mS/cm was measured at 120 °C. Moreover, the bisphosphonated membranes did not decompose at temperatures of up to 240 °C under air. The study also showed that high degrees of hydrolysis of the bisphosphonate units were crucial in order to reach the high proton conductivity and thermal stability necessary for fuel cell applications.