The morphology of an AB diblock copolymer comprising a highly biocompatible poly[2-(methacryloyloxy)ethyl phosphorylcholine] block and a pH-sensitive poly-[2-(diisopropylamino)ethyl methacrylate] block (PMPC30-b-PDPA(60)) was analyzed using SAXS and (S)TEM. PMPC30-b-PDPA(60) films cast from ethanol solution at room temperature exhibit a thermodynamically quasi-stable cylindrical morphology, which undergoes an order-order transition upon thermal annealing at 170 degrees C: the resulting lamellar structure coexists with a minor proportion of cylinders organized into a hexagonal compact phase. In contrast, copolymer films cast from methanol do not undergo the same morphological transition. Instead, short-range liquid-like structures are obtained regardless of the annealing processes. Finally, direct self-assembly to form a lamellar morphology at room temperature can be achieved by solvent-casting from aqueous solution at pH 4.