We have found that Ni3Al intermetallics shows catalytic activity for H2 production reaction, such as methane steam reforming. In addition to the single-phase Ni3Al, we recently studied the catalytic property of the Ni3Al/Ni two-phase alloy in foil form, and have found that the catalytic activity is not so high in the cold-rolled state. In case of atomized Ni3Al powder with a NiAl/Ni3Al two-phase structure in fact, it is possible to improve the catalytic activity for methane steam reforming by chemical pretreatment in acid and subsequent alkali solutions [1]. The reason was similarly due to the formation of fine Ni particles on the porous surface. These results show a possibility to improve the catalytic activity of Ni3Al/Ni two-phase alloy foils by the two-step chemical treatment similar as powder. In this study we carried out this chemical pretreatment on the surface of Ni3Al/Ni two-phase alloy foil and the catalytic properties for methane steam reforming were evaluated. Ni3Al/Ni two-phase alloy foil (Ni-18 at% Al) with a thickness of 30 μm was used. The chemical pretreatment consisted of two steps, the first step was acid leaching (HCl + HNO3, vol. ratio 3:1) and the second step was alkali leaching (NaOH solution, 20 wt%). Methane steam reforming was carried out in a conventional fixed-bed flow reactor as described in previous reports [1]. Prior to the reaction, the foil was reduced at 873 K for 1 hour by H2. Then isothermal test was carried out at 1123 K for 50 hours under steam-to-carbon ratio of 1. The surface morphology was analyzed using SEM and EDS. The chemically pretreated foil showed much higher catalytic activity for methane steam reforming than the non-treated foil. Especially, the catalytic activity rapidly increased during in the first several hours, and then slowly increased during the subsequent reaction. The maximum H2 production rate was about 30 L/min/m2. This value was 40 times higher than the non-treated foil. This result indicated that the catalytic activity can be improved by chemical pretreatment in acid and subsequent alkali solutions. The surface morphology observation revealed that acid leaching dissolved Ni3Al phase and alkali leaching dissolved Al mainly, resulting in the Ni-enriched surface structure. We consider that this Ni-enriched surface introduced by the two-step chemical treatment attributed to the initial catalytic activity. It also revealed that many fine Ni particles were formed on the surface during the reaction, and the amount of fine Ni particles increased with time. We consider that these Ni particles produced during reaction, contributed to the increase of catalytic activity during reaction. [1] Y. Ma, Y. Xu, M. Demura, D.H. Chun, G.Q. Xie, T. Hirano, Catal. Lett. 112 (2006) 31