5 20 minutes say 10 minutes it may be found that treatment of the ion exchange membrane with r 4 nx wherein the r group is lower alkyl i e c 1 c 3 preferably methyl gives outstanding results in terms of separation and flux it is a feature of this invention that the composite membrane used in the process of this invention may typically comprise i an optional carrier layer characterized by porosity and mechanical strength for supporting a porous support layer and a separating layer ii a preferred polysulfone porous support layer of molecular weight of 5 000 100 000 of thickness of 10 80 microns and of molecular weight mn cut off of 25 000 100 000 and iii as a non porous separating layer either a polyvinyl alcohol of molecular weight of 20 000 200 000 which has been crosslinked with an aliphatic dialdehyde containing 3 8 carbon atoms or b a high molecular weight ion exchange resin in membrane form bearing a pendant acid group which membrane has been contacted with a quaternary amonium salt containing four hydrocarbyl groups when the charge solution contains a component in which the polysulfone is soluble e g a ketone or an ester then the preferred support layer may be a polyacrylonitrile the membranes of this invention may be utilized in various configurations it is for example possible to utilize the composite in a plate and frame configuration in which separating.