Chlorine　exposure　is　one　of　the　most　commonly　encountered　challenges　that　cause　polyamide　(PA)　membrane　failure　in　use.　The　synergistic　effect　of　iron,　which　is　ubiquitous　in　water　environment,　with　chlorination　is　however　often　overlooked.　This　study　systematically　investigated　the　performance　of　thin　film　composite　(TFC)　nanofiltration　(NF)　membranes　under　dynamic　exposure　of　free　chlorine　and　Fe2+　in　filtration　process.　It　was　found　that　the　36%–70%　reduction　of　water　flux　after　chlorine-only　exposure　was　mainly　attributed　to　the　competing　effects　of　reduced　hydrophilicity　by　N-chlorination　over　decreased　crosslinking　by　chlorination-promoted　hydrolysis,　while　the　opposite　trends　of　salt　rejection　for　mild　and　severe　chlorination　were　believed　to　be　caused　by　the　competing　effects　of　decreased　crosslinking　and　increased　charged　density.　The　presence　of　Fe2+　in　the　feed　generally　resulted　in　a　higher　(or　equivalent)　water　flux　and　a　lower　salt　rejection　compared　to　chlorine-only　conditions,　owing　to　catalytic　oxidation　and　iron　deposition.　Fe2+-induced　catalytic　oxidation　by　hydroxyl　radicals　(·OH)　formed　by　the　reaction　between　free　chlorine　and　Fe2+　promoted　C–N　breakage　and　led　to　a　much　looser　separation　layer,　which　contributes　more　remarkably　to　performance　variation　(i.e.,　2.5–6.5　time　increase　of　water　flux　and　near-zero　salt　rejection　for　1000　mg/L　Cl2)　than　chlorination.　In　addition,　the　much　lower　normalized　salt　rejection　for　10　mg/L　Fe2+　than　0.1–1　mg/L　Fe2+　(10%–30%　vs.　100%–120%)　under　10–100　mg/L　Cl2　at　24　h　was　attributed　mainly　to　a　much　denser　iron　deposition　layer,　which　neutralized　the　membrane　surface　charge　and　decreased　the　electrostatic　repulsion　between　salts　and　membranes.　Hence,　the　combination　of　chlorination,　catalytic　oxidation　and　iron　deposition　was　proposed　to　be　the　mechanisms　synergistically　affecting　the　membrane　behavior.　©　2022　Elsevier　B.V.