Atomically ordered and clean Rh(111) surfaces were exposed to electrolyte solutions by the flame-annealing-quenching method. In situ scanning tunneling microscopy discerned atomic features with a (root 3 x root 7) symmetry on Rh(111) for adsorbed sulfate/bisulfate in 0.5 MH(2)SO(4) The ordered structure was consistently observed on the entire surface of Rh(111), including very narrow (111) terraces and near the end of the step edges. The adlayer structure observed on Rh(111) was found to be almost identical to that observed recently on Au(111) at positive potentials. A model structure is proposed to explain the nonuniform spacing in the unit cell of(root 3 X root 7). Hydrogen-bonded water chains are expected to exist along the root 3 direction between neighboring rows of the adsorbed sulfate. Bright and dark spots appearing in the STM images are assumed to arise from adsorbed sulfates and water molecules in the second layer of the first water bilayer.