The effect of thallium(I) ions on the adsorption of CN-containing species on a gold electrode from alkaline cyanide and dicyanoaurate solutions was studied by a radioactive labelling method. In cyanide solutions, adsorption of CN-containing species decreases linearly with the coverage of thallium adatoms in the potential range of thallium underpotential deposition. No adsorption of CN-containing species was observed on the bulk thallium layer formed below the Nernst potential for the couple Tl/Tl+. In dicyanoaurate solutions, the effect of thallium ions on the adsorption of CN-containing species depends strongly on dicyanoaurate and thallium bulk concentrations. At the lowest dicyanoaurate concentration (1 mM) and the highest thallium concentration (0.1 mM) investigated, the effect of thallium ions on the adsorption of CN-containing species is similar to that in cyanide solutions. However, at the highest dicyanoaurate concentration (5 mM) and the lowest thallium concentration (0.01 mM) investigated this effect is significantly lower than in the first case. The observed effects of thallium ions both on the adsorption of CN-containing species from dicyanoaurate solutions and on the kinetics of dicyanoaurate reduction on gold are discussed in terms of the thallium surface coverage on a gold electrodeposit.