Dilute aqueous solutions can be effectively concentrated with crosslinked partially hydrolysed polyacrylamide gels. Because the gels are near their critical point, they can be regenerated with small changes in pH. The selectivity of concentrating nonelectrolyte solutes can be adjusted by changes in gel crosslinking; the selectivity of separating electrolytes seems largely the result of Donnan equilibria. The regeneration of the gels is effected largely by altering the ionization of carboxylic acid groups within the gel, and less influenced by ion pairing. The regeneration is compromised at high salt concentrations which reduce the gel's swelling.