SMALL Metal clusters, containing of the order of 10 to 10,000 atoms, are of interest both for exploring the fundamental question of how atomic-scale properties develop into bulk properties and because such clusters are expected to have interesting optical, electronic and magnetic properties1. The expectation is that bulk-like metallic behaviour will become increasingly evident as the cluster size increases, but at what stage bulk properties appear is not yet clear. Here we present results of a study of the platinum cluster compound Pt309(Phen*)36O30+/-10 (refs 2-4), in which we use Mossbauer spectroscopy to investigate the bonding environment of the Pt atoms. Because Pt lacks a good Mossbauer isotope, we transform a fraction of the Pt-196 atoms in the clusters into Mossbauer nuclei Au-197 by neutron irradiation. The resulting spectra show that the inner (147-atom) core of the Pt cluster exhibits a metallic character like that in the bulk metal, whereas the surface atoms are not bulk-like. These bulk-like metallic properties may be acquired by metal-cluster cores as small as about 150 atoms.