Cu-SSZ-39　demonstrated　exceptional　resistance　to　hydrothermal　aging　during　NH3-SCR.　So　far,　the　precise　quantification　of　adsorption　sites　for　various　reactants　on　Cu-SSZ-39　remains　uncertain.　In　this　study,　the　quantitative　analysis　using　a　transient　response　method　(TRM)　allowed　for　determining　the　adsorption　quantities　of　reactants,　as　well　as　the　concentrations　of　various　Cu　species,　and　the　reaction　pathway　on　Cu-SSZ-39　and　H-SSZ-39.　Cu　benefited　N2　formation　through　NO　reacting　with　adsorbed　NH3,　while　NH4NO3　reacted　with　NO　to　form　N2　instead　of　NO2　or　N2O.　On　Cu-SSZ-39,　the　dominant　pathway　involved　NO　reacting　with　adsorbed　NH3,　resulting　in　the　production　of　N2　and　H2O,　and　another　important　pathway　was　＇NH4NO3　fast　path＇　(NH4NO3　+　NO　+　*-ONH4　+　*　→　2　N2　+　3　H2O　+　2　*-OH).　O2　facilitated　the　reaction　between　NO　and　NH3.　The　numbers　of　Z2Cu2+and　ZCu-OH　were　also　determined　by　TRM.　More　Z2Cu2+　species　caused　its　high　hydrothermal　stability.　©　2023　Elsevier　B.V.