a thin film composite ultrafilter / hyperfilter water softening membrane on a microporous substrate is produced by an interfacial polymerization reaction between an aqueous phase comprising a blend of water soluble and water dispersible compounds , the former containing piperazine and polyamines , the latter using a chlorine or other oxidant reacting compound which protects the resulting membrane from damage by any chlorine present and an organic phase comprising polyfunctional agents capable of reacting and / or polmerizing with one or more of the ingredients of said aqueous phase .