The structures of the Cu(111) and the Cu(111)(root 7 x root 7)R19.1 degrees-S reconstructions have been studied by surface X-ray diffraction and two new structural models are proposed. Although the unit cells for the two structures are quite different, a high degree of similarity has been revealed. It consists of a Cu4S unit with sulfur adsorbed in the fourfold hollow site on top of a Cu tetramer. The \4-141--- structure contains two Cu4S units, the (root 7 x root 7)R19.1 degrees structure only one. In both structures additional sulfur atoms are present. The sulfur coverages for the structures are 0.35 and 0.43 ML, respectively. The building block for the structures is similar to the Ni4S unit which we recently found in the Ni(111)(5 root 3 x 2)-S structure. We propose that the driving force for the reconstruction is the chemical stability of the Cu4S units given by the tendency of sulfur to be fourfold coordinated. (C) 1997 Elsevier Science B.V.