The　deterioration　of　separation　performance　of　polyamide　membranes　after　chlorine　exposure　is　a　core　issue　in　the　application　process　of　membrane　technology.　The　co-existence　of　metal　ions　(i.e.,　Al3+　and　Mn2+),　which　are　ubiquitous　in　different　types　of　water　bodies,　may　complicate　the　chlorination　process.　This　study　systematically　evaluated　membrane　degradation　under　the　simultaneous　presence　of　free　chlorine　and　Al3+　or　Mn2+.　The　membrane　water　flux　after　chlorine　exposure　of　2400　mg/L·h　was　decreased　by　70　%,　of　which　49　%　was　resulted　from　chlorine　exposure,　attributing　to　competing　effects　of　the　decreased　hydrophilicity　(contact　angle　increased　from　56°　to　68°)　and　cross-linking　degree　(Onet/N　ratio　increased　from　1.1　to　2.3).　The　changes　in　the　physiochemical　properties　of　membranes　were　due　to　chlorination　and　hydrolysis　induced　by　chlorination.　The　addition　of　Al3+　or　Mn2+　to　chlorine　resulted　in　a　further　reduction　of　8　%–13　%　for　water　flux,　and　a　variation　as　high　as　21　%　(increase)　and　15　%　(decrease)　for　salt　rejection　compared　to　virgin　membrane　at　different　exposure　periods.　These　cations　similarly　resulted　in　a　more　hydrophobic　membrane　surface　with　varied　surface　charge　and　the　formation　of　deposition　layers.　In　addition,　Al3+　and　Mn2+　did　not　affect　chlorination　degree　(chlorine　incorporation).　However,　Al3+　promoted　chlorination-promoted　hydrolysis,　while　Mn2+　inhibited,　reflected　by　the　opposite　trends　of　Onet/N.　©　2023　Elsevier　B.V.