The initial stages for Tl electrodeposition on Pt(111) covered with iodine in (root7 x root7)R19.1degrees or (3 x 3) structures are investigated by cyclic voltammetry, low energy electron diffraction and Auger electron spectroscopy. Estimation of faradaic charges in cyclic voltammetry is facilitated by simultaneous measurement of the double-layer capacitance by ac voltammetry. The first electrodeposition peak corresponds to the deposition of 0.22 ML Tl atoms, independently of the structure of the initial iodine adlayer. However, after the electrodeposition of Tl on Pt(111) (root7 x root7)R19. 1degrees-I, the surface undergoes an irreversible rearrangement to a Pt(111) [(4)(3) (0)(6)]-TlI structure, with two Tl atoms and six I atoms per unit cell. In this rearrangement, thallium and iodine are removed from the surface in a 1:1 ratio. (C) 2003 Elsevier B.V. All rights reserved.