In situ scanning tunnelling microscopy was carried out for the underpotential deposition (UPD) of mercury on Au(111) in sulphuric and perchloric acid solutions. In a sulphuric acid solution, a ((2)((1) over bar)(2)(1)) structure is observed at potentials more positive than the first UPD peak, which is assigned to be of the structure of adsorbed bisulphate ions. After the formation of the first UPD adlayer in the sulphuric acid which is assigned to be of the structure of adsorbed bisulphate ions. After the formation of the first UPD adlayer in the sulphuric acid solution, two different structures of ((20)(33/2)) and ((1(1)(4) over bar)(4)) are simultaneously produced on the same atomically flat terrace. On the contrary, a single structure of ((02)(21)) is found in the perchloric acid solution. The different structures formed in the different electrolytes indicate a large influence of the electrolyte anions on the UPD structures, It is clearly shown that the bulk deposition of mercury on Au(111) roughened the gold surface structure. The roughened surface is recovered as smooth after the stripping of Hg at 1.3 V vs. a reversible hydrogen electrode because of the surface diffusion of gold atoms.