Abstract: A process for producing a microporous plastic member useful as a battery separator which comprises(a) forming a coherent dough by mixing a vinyl chloride resin with a plasticizing amount of a first solvent and with finely divided particles of a filler which contains volatilizable constituent releasable therefrom by heating and which shrinks substantially and irreversibly upon release of said volatilizable constituent therefrom, an example of which is hydrated silica;(b) forming the solvent-containing dough into a shaped member;(c) extracting the first solvent from the member before any substantial evaporation of solvent occurs by contacting it with a second solvent in a liquid bath at a temperature substantially below the boiling point of any liquid present, the second solvent being one which is capable of dissolving the first solvent without dissolving the resin and the filler, thereby deplasticizing the member by extractively removing the first solvent from the member; and(d) thereafter heating the member a