The effect of NiO content in mesoporous NiO-Al2O3 catalysts on stability of the catalytic activity was investigated for partial oxidation of methane to syngas under the reaction conditions at 650 degrees C and 1 MPa over the contact time range from 0.1 to 5 ms. Mesoporous/porous NiO-Al2O3 catalysts with a high BET surface area were prepared with NiO content of 3.7-75 wt%. From XRD patterns and TPR profiles observed in these catalysts before H-2 pretreatment, the presence of strong interactions between NiO and Al2O3 was indicated, which gave relatively small NiO particles in the catalysts. As results of activity tests conducted by changing the contact time from 5 ms to lower until the catalyst lost its oxidation activity, the order of the catalyst tolerance against oxidation of nickel active species on them was: NiO(75)-Al2O3 > NiO(40)-Al2O3 > NiO(90)-Al2O3 > NiO(11)-Al2O3. As results of long runs and TG-DTA analysis, NiO(40)-Al2O3 showed the stable activity without any carbon deposition after 24h reaction. This is because the Ni particle size and dispersion are 5.9-7.1 nm and 10.8%, respectively, which are considered to be suitable to prevent both carbon deposition and oxidation of nickel species during reaction, while NiO(75)-Al2O3 with Ni particle size of 11.7-15.9 nm gave high carbon deposition after a long run. The Ni particle size of 10 nm was a borderline for carbon deposition under reaction conditions at 1 MPa. (c) 2011 Elsevier B.V. All rights reserved.