High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO(x) with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm-1 on Cu during reaction conditions (400-degrees-C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu+(-)CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.