AbstractWe used partial crosslinking of the pore-forming block of quaternized PS-b-P4VP integral asymmetric isoporous membranes as a tool to control the effective pore size in aqueous and alcoholic media. A versatile top-down membrane post-modification approach using a combination of difunctional and monofunctional alkyl halide was employed to obtain well-defined nanochannels containing polyelectrolyte brushes. Mild and scalable gas-solid reactions were used for the membrane post-modification. The permeance of the modified membranes changed systematically over a wide range, proving the potential of using the membrane post-modification technique to tailor the pore size from ultrafiltration to nanofiltration regime. Using a membrane post-modification technique, for the first time a series of integral asymmetric isoporous membranes having equal charge density but distinct effective pore size was successfully prepared. The separation performance based on size and charge is demonstrated by separating 1–2 nm organic molecules (having molecular weights in the range of 248–480 g mol−1) with excellent selectivity and permeance. For example, one of the prepared membranes showed a 2.4 times higher retention of methylene blue (MB+, 319.85 g mol−1) than chrysoidine G (CG+, 248.71 g mol−1) from an aqueous feed solution containing equimolar MB+ and CG + while the permeance was 134 Lm−2h−1bar−1.