Porous track membranes are produced by exposing a polymeric film to a
bombardment of heavy ions to provide the film with a track density, and
etching pores into the resulting tracked film with an etching solution to
provide the film with a density of the pores corresponding to the track
density under conditions to maintain turbulent flow. An alkaline etching
solution is used that contains salts of alkali metals in sufficient
concentration to increase the boiling point of the resulting
alkali-metal-containing solution to temperatures in excess of about 100
up to about 150.degree. C.