The effect of noble metals (M = Rh, Pt and Pd) added to NiO-MgO solid solution catalyst on catalytic activity and carbon deposition was investigated in oxidative steam reforming of methane under atmospheric and pressurized conditions. The noble metal addition enhanced methane conversion at low W/F condition, such as 0.13 g h mol(-1), and the promoting effect was easily observed even for a small amount of addition, such as 0.035% Rh. This noble metal addition can maintain Ni species in a reduced state and it can enhance the methane activation ability and catalyst reducibility. Under pressurized conditions, it was found that the addition of Rh and Pt decreased carbon formation almost completely on Ni0.2Mg0.8O. The catalyst characterization suggests that noble metals-Ni alloy particles with the surface segregation of noble metals are formed on M/Ni0.2Mg0.8O. This is related to high resistance to deactivation due to oxidation and carbon formation. (c) 2005 Elsevier B.V. All rights reserved.