The ordered structure of a 4-mercaptopyridine monolayer self-assembled on Au(111) from neutral aqueous solution has been investigated in neutral, acidic, and alkaline solutions under potential control by scanning tunneling microscopy (STM). We found that the monolayer changes its structure associated with the shift of acid-base equilibrium of the pyridine tailgroup. The structures are described as striped (p x root3)R30 degrees, where p is 5 at pH 1, and 12 at pH 5 and 10. High-resolution imaging reveals that the spacing between nearest-neighbor sulfur headgroups is about 0.21 nm at pH 1. The spacing is close to the typical S-S bond length in disulfides, suggesting that the molecule is adsorbed on the surface as a disulfide rather than an Au-thiolate. The disulfide-like species is stable only in strongly acid solutions and the molecule is adsorbed as an Au-thiolate in neutral and alkaline solutions. From the pH-dependent structural changes, we propose that the surface molecules are protonated one-dimensionally along molecular rows (stripes) and the fraction of the protonated molecular rows depends on solution pH.