This paper describes the formation of a new generation of organic solvent nanofiltration (OSN) membranes via one-step in-situ crosslinking method by casting a solution mixture of sulfonated polyphenylsulfone (sPPSU) and hyperbranched polyethylenimine (HPEI) through a non-solvent induced phase inversion process. Ionically crosslinking reactions took place between sPPSU and HPEI, and were confirmed by FTIR and XPS. The as-fabricated membranes showed good chemical stability in ethanol and isopropanol, and were evaluated in terms of pure ethanol permeability and rejection ability of different dyes with molecular weights ranging from 320 to 1017 g/mole in ethanol. The effects of HPEI concentration in dope solutions on OSN performance were firstly investigated, it was found that 0.3 wt% HPEI gave the highest permeance (7.96 L m(-2) h(-1) bar(-1)) and a rejection of 51% to Rose Bengal (Mw = 1017). To further improve the OSN performance, several post treatments including dip coating and pressure assisted coating were conducted using a 1.0 wt% HPEI solution in either water or ethanol. The coating under pressure showed the most effectiveness to enhance the crosslinking reaction and narrow the molecular weight cut off (MWCO). The resultant membrane had a MWCO of approximate to 600 g/mole, a rejection of 99.9% to Rose Bengal and an ethanol permeance of 1.47 L m(-2) h(-1) bar(-1). Compared to the unmodified sPPSU, all ionically cross-linked membranes possess much more stable rejections and permeance over a period of two weeks, especially for the membrane with the pressure assisted coating. These ionically cross-linked membranes may open up new strategies to design high performance OSN membranes due to the facile in-situ cross-linking method.