A composite palladium-gold membrane reactor (MR) was packed with a Ni-based catalyst to perform natural gas steam reforming for the purpose of producing high-purity hydrogen. In a related previous study, high hydrocarbon conversion (> 80%) and hydrogen recovery (similar to 65%) were reached at 450 degrees C and 300 kPa, with high-purity hydrogen obtained in all the experimental tests. The membrane has shown a hydrogen permeance of 1.30 x 10(-3) mol/sm(2) Pa-0.5 at 450 degrees C, and near infinite H2/Ar ideal selectivity at each operating temperature and a pressure of 50 kPa. In this study, the same membrane was used to investigate the influence of the reaction temperature and the gas hourly space velocity (GHSV) in order to optimize the membrane reactor performance. Specifically, the temperature and the GHSV were varied from 350 degrees C to 450 degrees C and from 1300 h(-1) to 7800 h(-1), respectively, while the reaction pressure and the steam-to-methane ratio were kept constant at 300 kPa and 3.5/1, respectively. In addition, a long-term natural gas steam reforming reaction was conducted for more than 260 h at different temperatures to evaluate the stability of the membrane reactor. The best MR performances were obtained at the lowest GHSV and highest temperature with greater than 80% hydrocarbon conversions and 80% hydrogen recovery. High-purity hydrogen was obtained in all of the experimental tests and no coke formation was observed. Finally, it is demonstrated that the Pd-Au MR, which was operated for 800 h under different reaction conditions, displayed stable performance, in terms of conversion, hydrogen recovery and product selectivity, while the membrane showed stable performance for more than 1000 h with minimal degradation.