Cd underpotential deposition (upd) on Au(1 1 1), from a sulfate electrolyte, has been investigated using electrochemical scanning tunneling microscopy (EC-STM), ultra-high-vacuum emersion techniques and electrochemical quartz crystal microgravimetry (EQCM). A number of atomically resolved adlattices were imaged with EC-STM and structures are proposed. Low energy electron diffraction (LEED) patterns observed after Cd upd consistently indicated a (root3 xroot3)R30degrees unit cell, which is closely related to those observed with in situ STNI. Comparisons of the EC-STM structures with the LEED results suggest that some change occurred upon emersion of the crystal from solution. EC-STM suggests that the Au(1 1 1)-(1 x 23) reconstruction was not lifted by Cd upd. Additionally, the morphology of the reconstruction dictated the structures formed by the adlayer on the surface. Specially, two structures were observed on the reconstructed surface, while others were observed in its absence. EQCM results suggest a large increase in mass upon Cd upd, which appears to result from co-adsorbed anions and possibly the formation of a CdSO4 bilayer. There was evidence that Cd upd results in a surface alloy. Near -0.5 V, small alloy nanoclusters appeared, distributed randomly across the surface in the absences of the reconstruction, and at domain walls in the presences of the reconstruction. (C) 2003 Elsevier Science B.V. All rights reserved.