Electrocrystallisation of thick (exceeding a few microns) gold layers from weakly acidic KAu(CN)(2) aqueous solutions is not feasible unless specific addition agents are present in the bath. Tl+ salts are empirically known to enhance the growth rate of gold. In this paper we investigate the action of Tl+ at the growth interface by electrochemical and spectroelectrochernical techniques and propose a mechanism for its action as a cathodic depolariser. The electrochemical behaviour of Tl+ was studied by linear sweep voltammetry, cyclic voltammetry, and differential capacitance measurements. The interactions of Tl+ and CN- at the growth interface were studied in-situ during the electrocrystallisation process by Raman and FT-IR spectroscopies. Cyanoaurate baths display a significant cathodic passivation, which is reduced by addition of Tl+. Voltammograms of baths containing Tl+ show underpotential deposition features but no stripping peaks. Spectroelectrochemistry shows that the deposition of Tl+ is specifically active in cathodically passivated areas. This behaviour is suggestive of an assisted An electrodeposition mechanism. Tl is oxidatively desorbed by Au(I), giving rise to the growth of thick, pure gold films. (C) 2002 Elsevier Science B.V. All rights reserved.