Poly(vinyl alcohol) (PVAL) was modified and insolubilized by reaction with potassium peroxydisulphate (K2S2O8) and used to make reverse osmosis (RO) membranes on tubular poly(arylether sulphone) (PES) ultrafiltration (UF) substrates. Prior to making the membranes, aqueous solutions of PVAL and K2S2O8 were aged. The effects of solution maturation times on the salt retentions and permeate fluxes of the resultant membranes were determined. Increased maturation times were found to lead to membranes with higher salt retentions. The K2S2O8-crosslinked PVAL membranes were also tested with an industrial effluent. Changes which took place in the maturing PVAL/K2S2O8 solutions were characterized by viscometry, UV spectroscopy, H-1- and C-13-NMR spectroscopy, and pH measurements. Only the most important results of these analyses are reported here.