The first stage of mercury underpotential deposition on Au(111) electrodes in 0.10 M H2SO4 containing 1.0 mM Hg2+ has been studied by synchrotron,X-ray scattering techniques including grazing incidence X-ray diffraction and specular crystal truncation rod measurements. An ordered coadsorbed structure of sulfate/bisulfate anions and Hg cations was found at potentials between the first and second Hg UPD peaks (+0.80 V less than or equal to E less than or equal to +0.88 V vs Ag/AgCl(3 M KCl)). The coadsorption structure was found to consist of a compressed Hg honeycomb lattice with the honeycomb centers occupied by sulfate or bisulfate anions. The compression of the lattice is likely due to the formation of mercurous (Hg-2(2+)) ions which have a much shorter Hg-Hg distance than that in frozen bulk Hg crystals. The net charge transferred under the first Hg UPD peak suggests that the chemical state of the species in the coadsorbed structure is likely Hg2SO4. Our results indicate that both the chemical state of the mercury cations and the nature of the anions are important in the resulting electrodeposited structures.