This work shows that it is possible to prepare a well-defined Ru(001) surface by annealing in an airtight quartz cell purged continuously with hydrogen gas. Cyclic voltammograms obtained in 0.1 M hydrofluoric and perchloric acid solutions contain clear redox characteristics, owing to the interfacial events of hydrogen adsorption/desorption and formation/reduction of an oxygen adlayer. High-quality in situ scanning tunneling microscopy (STM) atomic imaging reveals the hexagonal lattice of the Ru(001) substrate in perchloric acid solutions at the onset of hydrogen evolution. Irreversible adsorption of the (bi)sulfate anions from 0.1 M sulfuric acid at -0.1 V (vs Ag/AgCl) renders a highly ordered structure, identified as (root3 x root7)oblique by in situ STM. This arrangement of (bi)sulfate anions is the same as those found at the (111) surfaces of Au, Pt, Rh, Pd, Ir, and Cu electrodes, Potential excursion to 0.4 V results in a well-ordered (2 x 2) oxygen adlayer at the expense of the (root3 x root7)oblique (bi)sulfate structure. Iodine atoms are adsorbed more strongly than the (bi)sulfate and oxygen adlayer species, irrespective of the potential. In situ STM imaging unveils three iodine adlattices, including (root3 x root3)R30degrees, (5 x root3)rect, and domain wall structures, as the potential is increased from -0.1 to 0.4 V. Oxidation of iodide to iodine atoms occurs at potentials more positive than 0.5 V to produce multiple layers of iodine adatoms at Ru(001).