Poly(ethylene-b-styrene-b-ethylene) (ESE) triblock copolymer-based alkaline anion-exchange membranes (AAEMs) were synthesized and characterized to evaluate the effects of polydispersity and ion exchange capacity (IEC) on the structures and properties of semicrystalline block copolymer AAEMs. The ESE AAEMs with relatively low polydispersity (D < 1.9) formed long-range-ordered lamellar microstructures oriented primarily parallel to the membrane surface, which resulted in adversely rough membrane surfaces and low strain at break (<60%). In contrast, the ESE AAEM with a relatively high polydispersity (D approximate to 2.5) developed microphase-separated domains without long-range order and formed membranes with smooth surfaces and excellent strain at break (>350%). These polydispersity-driven changes to the long-range-ordered states of block copolymer membranes show that the microphase separation in block copolymers benefits the dimensional stability and conductivity, but the long-range ordering of microphase-separated domains in block copolymers is not necessarily beneficial for the overall properties of block copolymer-based membranes.