Abstract: A process of concentrating an aqueous polyvinyl-chloride emulsion employing a membrane device comprising a feed zone and a permeate zone, the zones separated by an ultrafiltration semipermeable membrane, which process comprises: introducing the polyvinyl-chloride emulsion to be concentrated into the feed zone; removing a concentrated polyvinyl-chloride emulsion from the feed zone and removing an aqueous fraction from the permeate zone; interrupting the process of concentration periodically when the flux rate of the process falls below a desired flux rate value; and contacting the semipermeable membrane with a solution containing a solvent, such as tetrahydrofurane, for the polyvinyl-chloride for a time sufficient to restore the flux value to its desired level, the semipermeable membrane employed characterized by being solvent-resistant to the solvent employed in contacting the membrane.