The mechanism of gold alloy electrodeposition from acid baths in the presence of a trace amount of Tl(+) was investigated using cyclic voltammetry, chronoamperometry and chronopotentiometry. Analysis of chronoamperometric measurements showed a progressive and instantaneous nucleation and growth mechanism at low and high cathodic potentials, respectively. The current time transients of hard gold baths containing Tl(+) showed underpotential deposition (UPD) features. The surface morphology of the resultant deposits was studied by scanning electron microscopy (SEM). The SEM studies showed that addition of cobalt leads to the formation of fine sized grains. Also with addition of thallium to the solution, enhancement and inhibition of the gold deposition were observed at low and high cathodic potentials, respectively. A bright, grain refined, coherent deposit with high hardness resulted from hard gold plating bath by addition of Tl(+). (C) 2010 Elsevier B.V. All rights reserved.