Experiments using vibrationally resolved ultraviolet photoelectron spectroscopy (UPS) provide evidence that dissociative chemisorption of N-2 becomes less stable than the molecular chemisorption on small clusters consisting of less than 10 metal atoms, which can efficiently dissociate diatomic molecules in the bulk form. This result is different from the generally accepted view that undercoordinated atoms are chemically more reactive. Our observation can be rationalized with a less efficient screening of the positive holes in smaller clusters created by a metal to adsorbate charge transfer, caused by the lower number of the directly neighbouring atoms. (C) 2003 Elsevier B.V. All rights reserved.