Hydrophilic porous polymeric membranes with high permeabilities, and processes for the preparation thereof are disclosed. Membranes may be prepared by including a preferably hydrophilic cross-linkable component such as PVP (either by inclusion into the polymer dope prior to casting, or coating or quenching cast membranes); and treating the polymeric microfiltration or ultrafiltration membrane with a crosslinking agent to cross-link said cross-linkable component. Preferred cross-linking agents include Fenton's reagent.