The influence of PtRu bimetallic particle size and composition on cinnamaldehyde selective hydrogenation has been investigated for the first time using well-defined catalysts based on carbon nanotubes support. Very high selectivity towards cinnamyl alcohol together with high activity have been obtained provided a high temperature treatment of the catalyst is performed. HRTEM, WAXS and EXAFS analyses permit us to conclude that the remarkable influence of this high temperature treatment on both activity and selectivity arises from different phenomena. First, a particle size and a structural effect have been evidence that permits to increase the selectivity. WAXS and EXAFS point the formation of alloyed PtRu nanoparticles. Second, the heat treatment allows the removal of oxygenated groups from CNT surface. This may increase the cinnamaldehyde adsorption capacity and decrease the activation barrier for diffusion of substrate and product on the CNT surface, thus contributing to an increase in the activity. (C) 2010 Elsevier Inc. All rights reserved.