The selective hydrogenation of cinnamaldehyde was studied over carbon nanofibers (CNF) supported platinum and ruthenium catalysts. The catalysts differed independently in their metal particle sizes and amount of acidic oxygen groups on the CNF surface. For the catalysts with oxygen on the CNF surface, the larger metal particles (similar to 3.5 nm) displayed the highest selectivity towards cinnamyl alcohol. Surprisingly, when the oxygen groups were removed from the catalyst surface, the smaller particles (similar to 2.0 not) exhibited the highest selectivity to cinnamyl alcohol. Also the hydrogenation activity increased for all catalysts after oxygen removal. A model is proposed to account for the role of the metal particle size and oxygen surface groups in the hydrogenation of cinnamaldehyde. (C) 2008 Elsevier B.V. All rights reserved.