Proton exchange membranes (PEM's) are currently under investigation for membrane water electrolysis (PEMWE) to deliver efficient production of the high purity hydrogen needed to supply emerging clean-energy technologies such as hydrogen fuel cells. The microblock aromatic ionomer described in this work achieves high mechanical strength in an aqueous environment as a result of its designed, biphasic morphology and displays many of the qualities required in a PEM. The new ionomer membrane thus shows good proton conductivity (63 mS cm(-1) at 80 degrees C and 100% RH), while retaining mechanical integrity under high temperature, hydrated conditions. Testing in electrolysis has shown good energy efficiency (1.67 Vat 1 A cm(-2) and 80 degrees C, corresponding to 4 kWh/Nm(3) of H-2), making this ionomer a potential candidate for commercial application in PEMWE.