A method of reclaiming spent aqueous brine solutions used in the regeneration of water-softening resins is disclosed. An aqueous chloride or other brine solution is acidified with HC1 to a pH of between about 0.5 and 6 and a soluble sulfate salt, preferably Na2SO4 is added, together with a precipitation inhibitor of the type polyacrylamide. Following such treatment, the spent brine is pumped at high pressure axially through a spirally-wound nanofiltration-type membrane device which is effective to remove at least about 90 % of the divalent hardness while allowing passage therethrough of at least about 90 % of the monovalent cations. As a result of such treatment, about 90 to 95 % of the volume of spent brine can be efficiently and effectively reclaimed in a form in which it is suitable for use again in regenerating water-softening resins or the like.