Single-crystal Ni electrodes were investigated by in situ electrochemical scanning tunneling microscopy (STM) under potential control in 0.05 M Na2SO4 at pH 3.0. On Ni(111), a Ni(111)-(1x1) structure was observed in high-resolution STM images acquired at cathodic potential. Under passivation by forming oxide layers, a hexagonal lattice with an interatomic distance of 0.29 nm was discerned, which was attributed to oxide layers of NiO(111) or Ni(OH)(2)(0001). On Ni(100), a square lattice with a p(2x2) of oxygen was observed even at cathodic potentials. The p(2x2) structure was converted to a quasi-hexagonal structure in the passive region due to the formation of bulk oxide films. Sulfur adatoms chemically attached on Ni(100) formed a c(2x2) adlattice, which was consistently observed even under the anodic dissolution of the underlying Ni substrate. The dissolution of Ni from the S-modified Ni(100) was found to proceed in the layer-by-layer mode, forming atomically flat terraces extending over large areas.