The adsorption of cytosine (C) on gold electrode has been studied to gain a fundamental understanding of the interaction between biomolecule and metal entity, which has important ramifications in the modern research on biological surface science. The characteristics of tautomerization and acid-base equilibrium of C at Au(111) electrode are intriguing and challenging to the modern study of electrified interface. Although this system has been studied by using in situ scanning tunneling microscopy (STM) and vibrational spectroscopy, many critical issues such as intermolecular interaction, substrate structure, and concentration of C are yet to be explored. High-resolution STM was used in the current study to probe the structure of C adsorbed on an Au(111) electrode in sulfuric and perchloric acids as a function of potential and [C]. C molecules arranged differently on the Au(111) – (1 × 1) substrate in 0.1 M H2SO4 and HClO4 containing 1 mM C, indicating that anion was coadsorbed with C admolecule. Adsorbed C was protonated at the negative potential, but deprotonated at the positive potential. Meanwhile, C admolecule reoriented from horizontal to upright configuration, as inferred from the ordered structures observed at different potentials. The surface coverage of C on Au(111), evaluated from the ordered structures, increased with more positive potential and grew into a multilayer film in 1 mM C in these acids.