We investigated the microphase separation behavior of well-defined poly(arylene ether sulfone)-b-polylactide (PES-b-PLA) diblock copolymers. PES was synthesized by the nucleophilic aromatic substitution polymerization of 4-fluoro-4′-hydroxydiphenyl sulfone potassium salt in the presence of an allyl-functionalized initiator, which follows a chain growth condensation polymerization mechanism. A hydroxyl group installed via a thiol-ene reaction was utilized as the initiating site for the ring opening polymerization of d,l-lactide, producing the target polymer. The polymers were further purified by preparative size-exclusion chromatography and analyzed by small-angle X-ray scattering with temperature variations from room temperature to 150 °C. The PES block was glassy in the employed temperature range, but the PLA chains provided sufficient mobility for ordering of the block copolymer when PES was the minor fraction. An order-disorder transition (ODT) with changing temperature could not be located because PLA was not stable above 170 °C. From the degree of polymerization values of the polymers near the ODT, the Flory–Huggins interaction parameter, χ, could be roughly estimated as 0.12 at 150 °C. This high χ value suggests that engineering plastic-containing block copolymers could be useful in advanced lithographic and filtration applications.