Organic monolayers of 4-mercaptopyridine (PySH) and bis(4-pyridyl) disulfide (PySSPy) self-assembled on an Au(lll) electrode in perchloric acid solution were studied using electrochemical scanning tunneling microscopy (STM). Both monolayers formed from PySH and PySSPy exhibited the same surface feature on Au(111), appearing as a ladderlike structure. An ordered structure with intermolecular distances of 0.49 and 1.42 nm was consistently observed for the PySH and PySSPy monolayers describable as Au(111)-SPy, which was explained by a rectangular unit cell of p(5 x root 3R-30 degrees). High-resolution STM images revealed the internal structures and micro-orientation of chemisorbed pyridinethiolate molecules. They were oriented with the molecular plane of the pyridine rings mostly perpendicular to the surface, but the molecular axis considerably tilted with respect to the surface normal. It was also found that intermolecular interactions through the sulfur portions were thought to exist between two pyridinethiolate molecules, as is expected to be found in dimer-state adsorption. Thus, we report, for the first time, the detailed investigation of pyridinethiolate monolayers on Au(111) in solution at the molecular level.