In this study we report changes in the glass transition temperature (T g ) of the polyamide barrier layer (PBL) of a commercial RO membrane as a function of chlorine treatment in which concentration and pH were systematically varied. The results indicate a monotonic decrease of T g with increasing exposure time at a given chlorine concentration. Further, both the degree and the rate of T g decrease vary significantly with both the chlorine concentration and pH value of the aqueous chlorine solution. Moreover, the decrease in PBL T g correlates well with the reduction in the salt rejection of the membrane, but the magnitude of the correlation is specific to chlorine concentration and pH values.