Hydrophilic polymeric phase inversion membrane

A process for rendering hydrophilic a separation membrane having a hydrophobic matrix polymer comprising preparing a membrane solvent system, said system comprising a hydrophobic bulk matrix polymer which absorbs less than 0.5% of its weight of water over a 24 hour period and 4% or less of its weight of water at equilibrium, and a block copolymer wetting agent having hydrophobic and hydrophilic polymer units, and doping said membrane solvent system with .gamma.-butyrolactone additive which is compatible with the bulk matrix polymer and which is capable of positioning the hydrophilic portion of the block copolymer at the matrix-solvent interface during membrane preparation, such that the resulting membrane is substantially instantaneously wetted as compared to the wetting rate of the same membrane solvent system without the .gamma.-butyrolactone additive.