The structure of 10 Å platinum particles encaged in Y-type zeolite has been determined by the radial electron distribution method ( RED) from X-ray diffraction data. When the particles are free from adsorbates the atom packing is distorted and the interatomic distances are contracted with respect to f.c.c. structure of bulk platinum. When the particles are covered with dissociated hydrogen there is a relaxation toward the normal f.c.c. structure. Carbon monoxide adsorption at 300 K on bare particles leads to an average f.c.c. lattice with normal bond lengths but there is a displacement disorder of the platinum atoms around the nodes. Therefore the actual Pt-Pt distances are probably contracted and elongated with respect to those of the average lattice. Similar observations in molecular platinum carbonyl clusters could provide good models for these particular defects appearing in response to CO bonding. Adsorption of hydrogen on CO covered particles does not remove the CO molecules whereas CO adsorption on H 2 covered particles displaces the hydrogen atoms.