AbstractUsing the example of an integral‐asymmetric isoporous membrane prepared from polystyrene‐block‐poly(2‐hydroxyethyl methacrylate) (PS‐b‐PHEMA), physical and chemical ways of post‐treatment are introduced with the aim to tailor the pore size. These post‐treatments are i) thermal annealing and ii) urethane chemistry of ethyl isocyanate (EI) in the presence of perfluoro(methyl cyclohexane). Via these approaches, the pore size of PS‐b‐PHEMA membranes is successfully tailored in the range of 10–20 nm with narrow pore size distribution by controlling the duration of thermal annealing and chemical reaction, respectively. The excellent hydrophilicity of the PS‐b‐PHEMA membrane is not changed by thermal annealing. The chemical postmodification using EI is associated with a loss of hydrophilicity with increasing conversion.