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In the present study, FeOOH/γ-Al 2 O 3 catalyst was first introduced into dielectric barrier discharge (DBD) plasma-catalysis system for toluene degradation, and its catalytic performance and mechanism were studied. It was found that the toluene degradation efficiency, energy yield (EY), and CO x selectivity were significantly improved in the plasma-FeOOH/γ-Al 2 O 3 system by comparison to the plasma-only process. The ozone concentration detected in this study decreased significantly in plasma-FeOOH/γ-Al 2 O 3 system, which could be concluded that the ozone catalytic process played an important role in the plasma-FeOOH/γ-Al 2 O 3 system for toluene degradation. The toluene and intermediates adsorbed on the surface FeOOH catalysts could react with O* from O 3 decomposition and O ads derived from oxygen vacancies, which was conducive to an improved degradation and mineralization performance. Besides, the transformation of O 3 to·OH on FeOOH surface was considered as one of the main mechanisms for enhancing toluene degradation when the relative humidity of the background gas was increased from 30% to 50%. The key process parameters, including specific energy input (SEI), catalyst loading, and oxygen content of the background gas, were optimized by response surface methodology (RSM) integrated Box-Behnken design (BBD). The results showed that oxygen content of the background gas was the most significant factor affecting the degradation efficiency of toluene, while energy yield was mainly determined by SEI. The possible toluene degradation pathways in plasma-FeOOH/γ-Al 2 O 3 system were proposed according to the identification of intermediates using GC-MS. • Toluene degradation was improved using FeOOH/γ-Al 2 O 3 catalysts in a dielectric barrier discharge (DBD) plasma reactor. • The possible mechanism for toluene degradation by plasma-FeOOH/γ-Al 2 O 3 system was proposed. • The catalyst enhanced toluene degradation through the transformation of O 3 to·OH. • The response surface method was used to optimize the experimental operating parameters. [ABSTRACT FROM AUTHOR]