Abstract: Vinylidene fluoride polymer ultrafiltration membranes are prepared by casting a sheet of said polymer dissolved in a mixture of a specified solvent and a specified non-solvent, on a smooth substrate, evaporating a portion of the solvent from the sheet, immersing said sheet in a gelation liquid therefore, and optionally, stabilizing the gelled sheet by heat treatment thereof. A porous vinylidene fluoride polymer membrane having smooth, unwrinkled surfaces can be prepared in accordance with the above described process without restraining the membrane during the evaporation and gelation steps by utilizing triethyl phosphate as the solvent.