Electrochemical measurements and in situ scanning tunneling microscopy (STM) are performed to establish a structure-reactivity correlation for peroxide or dioxygen reduction on underpotentially deposited (upd) TI on Au(111) in 0.1 M HClO4. At the potential of catalytic activity toward the Oz reduction, STM reveals the presence of Tl islands on the Au(111) terrace with height Of 0.24 +/- 0.03 nm. These islands expand in size and number; as the potential becomes more negative, but the full TI monolayer formed at -0.2 V vs NHE is catalytically inactive. Ethanethiol(EtSH) significantly inhibits the H2O2 reduction, and the kinetics and thermodynamics of EtSH adsorption on the Tl upd are quantitatively analyzed. STM shows that EtSH introduction leads to the formation of a 0.15 nm high terrace along the edges of the TI islands. This terrace is assigned to EtSH bound to the Au surface near the Tl islands with the alkyl chain oriented roughly perpendicular to the surface. These results show that edge sites around the Tl island are the active site of catalytic O-2 reduction by Tl upd on Au(111).