In-situ scanning tunneling microscopy (STM) and cyclic voltammetry have been used to examine chemisorbed Br adlayers on a well-defined Pt(111) surface in a 10 mM KClO4 solution (pH 4) in the presence of 3 mM KBr. The adlayer structures of the chemisorbed Br were found to be potential dependent. Asymmetric and hexagonal (3 X 3)-Br (theta(Br) = 0.444) structures were observed at potentials in the double-layer charging region. These two structures seemed to appear randomly on the Pt(111) surface. No clear transformation between them was observed. Atomic STM images observed at potentials slightly negative with respect to the first characteristic sharp peak were not consistent with the previously proposed model for a (4 X 4) structure. All Br atoms were aligned parallel to the Pt(111) lattice and appeared with an equal corrugation height. Incommensurate-like Br adlayers were believed to form at potentials more negative than the first characteristic sharp peak. The Pt(111)-(1 X 1) structure was clearly discerned in high resolution STM images at potentials near the hydrogen evolution potential. An identical structure was observed in HClO4 and H2SO4 solutions.