AbstractA large pore size distribution of conventional ultrafiltration membranes prepared by nonsolvent induced phase separation is a major limiting factor for separation of proteins having identical molecular weights e.g. bovine serum albumin (BSA) and hemoglobin (HB). Separation of this protein pair is performed at the isoelectric point of one protein by taking advantage of the electrostatic repulsion of the other protein with the membrane. Here we demonstrate the potential of designing a membrane with functional isoporous channels to perform the separation without relying on the electroneutrality of one protein. The integral asymmetric isoporous membrane was prepared from a polystyrene-block-poly(2-hydroxyethyl methacrylate-ran-2-(succinyloxy)ethyl methacrylate) (PS-b-P(HEMA-r-SEMA)) by a method which combines solvent evaporation induced self assembly and nonsolvent induced phase separation (SNIPS). The membrane having random –OH and –COOH groups along the pore wall showed an unprecendented ideal selectivity of BSA over HB, 𝜓𝐵𝑆𝐴/𝐻𝑏 at constant pH 7.4 where both proteins are negatively charged. To benchmark this result we have compared the 𝜓𝐵𝑆𝐴/𝐻𝑏 of 22 commercial and inhouse prepared membranes under similar condition.