Polyamide　(PA)　membranes,　widely　used　for　reverse　osmosis　and　nanofiltration,　are　prone　to　bromination　under　chlorinated　bromide-containing　water　conditions.　Conventional　wisdom　generally　assumes　HOBr　as　the　only　active　brominating　agent　responsible　for　PA　membrane　degradation,　while　some　more　reactive　but　less　abundant　brominating　agents　(including　Br2O,　BrOCl,　BrCl　and　Br2)　under　these　conditions　are　often　overlooked.　The　current　study　addresses　this　critical　literature　gap　by　systematically　evaluating　membrane　degradation　under　various　[Br−],　[Cl−]　and　[HOCl]　conditions.　The　observed　pseudo-first-order　rate　constant　of　membrane　degradation　(kobsm,　using　change　in　water　flux　as　a　surrogate　indicator)　was　found　to　be　well　correlated　to　[Br−],　[Cl−]　and　[HOCl]　(R2　>　0.90).　The　gradual　increase　of　[Cl−]　and　[Br−]　transforms　the　predominant　brominating　agent　from　HOBr　to　BrCl　and　Br2,　respectively,　under　excessive　[Br−]　conditions.　The　species-specific　second-order　reaction　rate　constants　followed　a　decreasing　order　of　kBrClm(2.6　×　104　M−1·s−1)　>kBrOClm　(2.0　×　103　M−1·s−1)　>kBr2Om(9.6　×　102　M−1·s−1)　>kBr2m(1.5　×　101　M−1·s−1)　>　kHOBrm(5.4　×　10−1　M−1·s−1).　Additional　decay　tests　using　benzanilide　(BA)　as　a　surrogate　monomer　compound　confirmed　BrCl　as　the　most　reactive　species.　Under　typical　seawater　conditions　(pH　8.0),　the　more　reactive　but　less　abundant　BrCl　had　significantly　greater　contribution　to　membrane　degradation　(85　%)　than　HOBr　(3　%).　Under　typical　neutral　wastewater　conditions,　both　BrCl　and　HOBr　contributed　equally.　The　current　study　developed　a　novel　characterization　technique　to　assess　membrane　degradation　by　determining　the　kinetics　of　the　oxidant-PA　reactions.　©　2022　Elsevier　B.V.