In situ electrochemical scanning tunneling microscopy (STM) disclosed that highly-ordered adlayers of adsorbed sulfate/bisulfate with atomic features having a (root 3 x root 7) symmetry formed on a well-defined Pd(111) surface in 10 mM H2SO4 in the double layer potential range. Well-defined Pd(111) surfaces were prepared by a flame-annealing-quenching technique. High resolution STM imaging revealed the structure of water molecules in both the first and the second layers of a water-bilayer. Small reversible peaks at 0.45 V versus a reversible hydrogen electrode were found to be due to a disorder-order phase transition. The Pd(111)-(1 x 1) and (root 3 x root 7) structures were observed at potentials negative and positive with respect to the small peaks, respectively.