Thin film composite (TFC) reverse osmosis (RO) membranes enjoy widespread use in desalination, but their sensitivity to oxidizing agents such as chlorine remains a continuing challenge. In contrast to many reports on the chemical aspects associated with decreased membrane performance after chlorine exposure, studies on the fundamental physical properties of the polyamide barrier layer (PBL) of TFC membranes are scarce. This omission is mostly due to the lack of techniques capable of characterizing such interfacially polymerized PBLs, which are ultrathin and insoluble. The focus of this study is the development of an AFM-based nano-thermal analysis technique that provides the first-ever result for the direct measurement of the glass transition temperature (T-g) of the PBL on several commercial TFC RO membranes. Moreover, the technique is utilized to study the changes in T-g of the PBL after exposure to chlorine solutions as a function of concentration and duration at constant pH. Results indicate significant and systematic reduction in T-g of the PBL with increasing chlorine concentration and exposure time. (C) 2011 Elsevier Ltd. All rights reserved.