The aim of this work was to study the effect of monochloramine solutions on polyamide membranes in the reverse osmosis process. Chloramines could represent a good alternative to replace free chlorine in the water disinfection process, resulting in higher water quality and minor environmental impact. The experiments were carried out in a bench-scale reverse osmosis unit using highly concentrated monochloramine solutions and two different commercial reverse osmosis polyamide membranes. The monochloramine solutions were prepared with an excess of ammonium chloride and commercial sodium hypochlorite in order to assure predominantly monochloramine formation with total consumption of free chlorine. Besides the iron that always was present, experiments were also conducted by adding aluminum to the chloramine solution, to determine whether this substance can act as a catalyst in the membrane degradation process. The membrane performance was evaluated before and after exposition to the oxidative solutions by using water and 2000 ppm concentrated NaCl solutions. It has been verified that the permeate flux increases continuously with the exposition time of the membranes to the monochloramine solutions. The salt passage also presented some changes, however monochloramines seems to be less aggressive than free chlorine and chlorine dioxide under similar operating conditions. Further systematic studies are required to determine precisely the effect of iron or aluminum on the membrane degradation process. (c) 2006 Elsevier B.V. All rights reserved.