A process for producing a porous polymeric structure which process comprises preparing a solution comprising a polymer which comprises vinylidene fluoride in a solvent/non-solvent mixture; holding the solution at an elevated temperature until the polymer is completely solvated; casting the solution to form a thin layer; and drying the thin layer to form a membrane. The polymer is dispersed in the non-solvent before addition of the solvent, when preparing the solution, to prevent microgel formation. The prolonged solvation at elevated temperature provides a stable solution, and enables membranes of controlled porosity to be formed.