The chemisorption of benzene at well-defined Pd(1 1 1) electrode surfaces in 0.1 M HClO4 has been studied by a combination of electrochemistry, molecular resolution scanning tunneling microscopy, and high-resolution electron energy loss spectroscopy. At potentials within the double-layer region, a well-ordered Pd(1 1 1)-c(2root3 x 3)-rect-C6H6 adlattice was formed in which the adsorbed benzene molecules are slightly tilted with respect to the metal surface. When the potential was decreased to values just inside the hydrogen evolution region, a third of the chemisorbed molecules were desorbed and a new ordered adlayer, Pd(1 1 1)-(3 x 3)-C6H6, was generated; in this less-densely packed phase, the coadsorption of water was indicated, and the aromatic molecules were chemisorbed more closely parallel to the surface. The potential-dependent structural and compositional transitions were found to be reversible. Structural models of the c(2root3 x 3)-rect and (3 x 3) adlattices are proposed. (C) 2003 Elsevier B.V. All rights reserved.