p-Diiodobenzene dissociatively chemisorbs on Cu{111} into three products-two iodine atoms and a surface-stabilized phenylene intermediate-at room temperature. The phenylene intermediates are mobile on copper terraces at 77 K, but can be trapped at atomic steps and impurity sites. Because of their rapid motion, no stable intermediates are observed by the scanning tunneling microscope on clean Cu{111} terraces. The I atoms remain in pairs on the surface, presumably at the site of dissociation. Initially, the phenylene intermediates preferentially adsorb at monatomic step sites. After saturating the step sites, additional phenylene intermediates adsorb at impurity sites on the atomic terraces.