In situ reflection infrared spectroscopy with electrochemical modulation has been used to investigate the coadsorption of anions for the system Tl-ads/Pt(111) in dilute sulfuric and perchloric acids. For the first time evidence of coadsorption of perchlorate induced by underpotentially deposited Tl atoms is reported. The spectroscopic data show that the platinum surface is covered by Tl adatoms and by perchlorate or bisulfate from E = 0.4 V up to the onset of the Tl stripping peak. The possible structures of the Tl-ClO4 and Tl-HSO4 adlayers at Pt(111) are discussed.