Accelerating effect of coexisting metal ions on the polyamide (PA) reverse osmosis membrane degradation by hypochlorite was evaluated. The acceleration mechanism by the metal ions was also examined. The acceleration of membrane degradation by hypochlorite was caused by all monovalent and divalent metal ions used in this study: Na+, K+, Ca2+, Mg2+, and Ba2+. Potency of divalent metal ions in PA membrane degradation was shown since divalent ions in much lower concentration than the monovalent metal ions caused membrane degradation. Na+ did not accelerate the degradation of the PA membrane in concentrations less than 100-mM while Mg2+ and Ca2+ showed no threshold limits. Accelerated membrane degradation in the presence of both monovalent and divalent metal ions seemed to be influenced by the threshold limit of the monovalent ion within the concentration range of each of the ions present. Membrane degradation below the threshold limit is dominated by the divalent ion while above the threshold limit the monovalent ion dominated the degradation. The degradation mechanism of PA membranes with or without metal ions is the same. However, in the presence of the divalent ion, it possibly acts as a catalyst in the amide hydrolysis, leading to accelerated membrane degradation. (C) 2015 Elsevier B.V. All rights reserved.