The investigations are related to some problems that arose in a demineralization water plant located in southern Brazil: membrane degradation, and decline of membrane performance after cleaning. The water pretreatment of this plant utilizes aluminum sulfate in order to coagulate suspended solids, and gaseous chlorine proceeds the disinfection. The chemical cleaning procedure of the membranes adopted in the industrial process uses two solutions, an acid and an alkaline prepared with sodium hydroxide and sodium laurel sulfate. This paper details bench-scale experiments used to determine the harmful effects of aluminum in catalyzing free-chlorine polyamide membrane oxidation. The study of the effect of the alum on chlorine membrane oxidation was undertaken by comparison with that when iron is present since both metals have several similarities. The experiments were carried out in a RO unit, utilizing commercial polyamide membranes and feed solutions with 500 ppm of aqueous chlorine with different concentrations of aluminum or iron (0.05, 0.1 and 0.26 ppm). This high chlorine concentration was used in order to evaluate its effect on membranes in a short period of time. Permeate flux and salt retention were measured during the experiments to determine the effect of chlorine on the performance of polyamide membranes. The experiments with residuals of alum or iron were performed under the same operating conditions and the results obtained compared qualitatively. Additional experiments were conducted to evaluate the effect of sodium laurel sulfate during the cleaning procedure of non-degraded and degraded membranes, and it was observed that the performance of degraded membranes diminished after cleaning.