We present evidence that adsorption probabilities (gas-surface adsorption dynamics) of CO depend distinctly on the Au cluster size, with a reactivity maximum at similar to 3 nm. This cluster size is consistent with the legendary reactivity enhancement seen for the CO-oxidation reaction. Electron beam lithography has been used to fabricate some of the samples (down to 12 nm) as well as physical vapor deposition. (C) 2009 Published by Elsevier B. V.