Abstract: A crosslinked polymeric polyvinyl sulfate membrane or crosslinked copolymer polyvinyl sulfate and polyvinyl alcohol membrane, or Nafion® or polytetrafluoroethylene suitable for use in an acid environment, and its use for recovering or reconstituting acid from a feed mixture comprising predominantly acid and water, the method comprising: processing said mixture using a first polymeric membrane such as Nafion® or polytetrafluoroethylene (“PTFE”) for example, to form a first retentate containing a substantially greater concentration of acid than said feed mixture and a first permeate containing a substantially greater concentration of water than said mixture, said first polymeric membrane selectively excluding permeance of the acid over the water found in the mixture, and recovering the first retentate; said first retentate can be processed further using a second water reduction membrane system mean to form a retentate stream containing a substantially greater concentration of acid than the first retentate and a