The reaction of acetaldehyde with pure Pd (in single crystal form. Pd(1 1 0) and Pd(1 1 1)) and with nanoparticles of Pd supported on silica have been investigated. The main reactions involved are decarbonylation (to produce CO and methane) and dehydrogenation (to produce CO, H-2 and carbon), and both of these reactions are seen on the unsupported and supported forms of Pd. In general the behaviour is similar, but there is a marked contrast in the temperature dependence of the reaction on single crystals and on the catalysts. On the crystals there is a sharp transition between decarbonylation and dehydrogenation, occurring at 420 K. whereas the catalysts show little activity for dehydrogenation, even up to 600 K. The reason for this relates to the capacity of the samples to absorb the carbon product of dehydrogenation into the bulk of the sample. The single crystal samples have a very low surface:bulk ratio and C formed at the surface is quickly lost to the bulk above 420 K, leaving an apparently clean and reactive Pd surface. The catalysts on the other hand, due to their high surface:bulk ratio, quickly become carbidised and are thereby passivated towards dehydrogenation, favouring high selectivity to decarbonylation. This proves that Pd catalysts are dehydrogenation catalysts and are only more selective to other routes when passivated by bulk carbon (or other additives such as Au). (C) 2010 Elsevier B.V. All rights reserved.