The effect of the cobalt particle size in the ethanol oxidative steam reforming reaction for hydrogen production was investigated using cobalt on carbon nanofiber catalysts. The smallest (< 4 nm) particles showed a significantly larger amount of acetaldehyde and smaller amount of H(2) as compared to larger ones. This result indicated that the C-C cleavage is more difficult on small Co particles under OSR conditions, which was due to a higher extent of surface oxidation of smaller particles in this reaction. The catalysts with larger Co particles (> 4 nm) were quite stable during OSR reaction but significant carbon formation was detected. (C) 2010 Elsevier B.V. All rights reserved.