The underpotential deposition of Tl on Au(111) in 0.1 M HClO4 solution containing 1 mM Tl+ and 1 mM Br- has been investigated by using cyclic voltammetry, in situ X-ray scattering, and rotating disk-ring techniques. Three interesting phenomena for the UPD of metals in the presence of adsorbed anions are reported, viz., (i) an increase of the underpotential shift for deposition of Tl on Au(111) in the presence of Br- in solution, (ii) a formation of two mixed adlayers, surface compounds, with the stoichiometry Tl-Br-2 and Tl-Br, and (iii) an exceptionally complex structural behavior of this system with five ordered phases. Bromide and thallium form incommensurate rotated-hexagonal close-packed phases at ca. 0.85 and -0.3 V, respectively. In the intermediate potential region three mixed phases exist, identified as 3TlBr(2)-(root 13 x root 13), 2TlBr-(3 x root 3), and TlBr-c(p x root 3).