It is shown that at high O-2(-) coverage on Ag(100) photo dissociation by soft X-rays generates oxygen atoms of covalent and of oxidic nature. The former species is obtained also by thermal dissociation and is the only observed product when irradiating at low O-2(-) coverage. The latter one is produced only photochemically and assigned to an Ag oxide. Them fragments produced in the photochemical process end up eventually in identical or in different sites, depending on the presence of other O-2(-) molecules in the immediate neighbourhood. (c) 2005 Elsevier B.V. All rights reserved.